Popular Science Monthly/Volume 43/October 1893/A Characteristic Western Plant Group
|A CHARACTERISTIC SOUTHWESTERN PLANT GROUP.|
A CURIOUS fascination gathers round any type of plant life that stands alone, as peculiarly characteristic of some one region of the world; and still greater does the interest become when we find, instead of a single type, an extensive group of closely related types holding a thus isolated position, and constituting a flora of themselves apart from surrounding plant realms. But such instances are rare—their very fewness primarily accounts for the impression they make upon both scientist and general
observer. In one corner or another of every continent botanists have found oddly specialized floras, distinct in aspect and purpose from the general run of vegetable forms. Many of these cases are of insignificant importance, save in their immediate interest to the specialist; some attract greater attention, as filling an especially noticeable gap in the series of plant relationships; a few become of widespread interest not only through unique specialization of structure, but also by virtue of their holding a really extensive and vitally important place in the economy of Nature. Of these last few one instance rises up most prominently of all, certainly without a full parallel elsewhere in the field of botanical science—and do we realize that it stands almost at our doors? It is the great three-typed plant group that forms the major part of
the flora of the far Southwest, on the arid plateaus and plains and rocky mountain heights of Arizona and New Mexico, western Texas and southernmost California, and over the boundary line far into northern Mexico. Here is the fatherland and here the supreme province of those three marvels of plant life—the cacti, with their strangely specialized vegetative body; the agaves, to which popular tradition has attached an epithet of fitting dignity in the name of "century plants"; and, thirdly, the yuccas, which claim, in addition to their floral splendor, the distinction of manifesting the interdependence of the flower and insect worlds with probably more striking force than obtains in any other single instance throughout the range of flowering plants.
Only within the memory of men still in the prime of life has the full significance of this Southwestern flora dawned upon the world of science. Far back in the history of early explorations travelers and naturalists had recognized the odd character of these north-Mexican plant forms, but to realize their inward meaning required the elaborate monographing of Engelmann and the broad generalizing of Asa Gray. For, strange as it may seem, one investigator after another, enthusiastic over the rich flora spread in such profusion from our Atlantic seaboard westward beyond the Rockies, nevertheless shunned, because of the many difficulties presented, this threefold group of Southwestern vegetation. Yet this, above all else, was a flora peculiarly American—originating, so far as we have yet been able to discover, on American soil, and belonging to America alone. So here there was a prospect of opening up to science a new aspect of plant life, and in due season the men came with the opportunities and inclination to accomplish the task. Foremost of all, and more than all the rest, stood forth the St. Louis physician. Dr. George Engelmann, a skilled man of medicine, with botanical inspiration. In him there seemed to be an especially keen appreciation of the opportunity offered for vastly aiding the cause of botanical science by the systematic study of little-known groups of plants; and through labors of this nature, in addition to his note as a physician, he placed his name among the greatest of monographers in the annals of botany. And to him belongs the credit of turning the full light of science upon the cacti, the agaves, and the yuccas, while through his investigations of these types the attention of our great American systematist, Asa Gray, was first directly turned to the vegetation of the Southwestern highlands. One of the absorbing problems of Gray's life-work was what he once fitly termed "botanical archæology"—the study of the geographical sources of our wealth of flora, and of the paths by which it had passed from one region to another. Years of experience had enabled him to propound the masterly theory of the great wave of ancient plant life sweeping down from the north and giving to the Old World and the New floras that have so many types in common. But later, largely in the light of Engelmann's revelations. Gray was brought to fully realize that a second great source of the peculiar elements in our flora lay in the Southwest, down on to the Mexican plateau, and beyond the reach of the influence of the Glacial age. Here was the possible source of a vegetation strictly American, and to it might be traced many now widely scattered tribes, but particularly and most obviously the three unique types we are especially considering. These have come down to us, in the land of the Aztec, as the descendants of an American flora whose traces are lost in far-off geologic ages, even as the forefathers of the Aztec are shrouded in the mist of prehistoric centuries. In truth, there seems a striking fitness in associating these odd monarchs of the soil with the barbaric majesty of the empire of Montezuma; a John Ruskin might say, the cactus typified the Aztec's sturdy, unwithering energy and stoic cruelty; the agave, his lofty nobleness of mind; and the yucca, the passionate beauty of his nature. But let the sentiment stand—the Aztec has passed away, and yet
this plant group still holds its own over the rocky hills and mountain sides and barren plateaus, withstanding drought and burning sun, and thriving in the arid sand wastes. And out from their native region many representatives have found their way southward, over into the West Indies, down through Central America, still further to the northern Andes, and almost to the Amazon; and others, though fewer, have come up into our Western plains and mountains, scattered over the Mississippi Valley, and passed through the Gulf States and far up the Atlantic coast. Thus eastward and northward from Texas we can count perhaps a dozen cacti, several yuccas, and one or two agaves, all luxuriating in their adopted habitats. Such, then, is a general suggestion of the position this plant group holds in our American flora. Let us now outline the relations of its three members to each other and to other flowering plants in general.
It is worthy of note that the three types referred to bear no close relationship to one another; on the contrary, they stand in distinct and rather parallel classes, and each respectively among the most perfect developments of its class. The cacti, on the one
hand, hold a place among the most highly organized of dicotyledons; while the agaves and yuccas belong in the other great class of angiospermous flowering plants, nearly parallel, but lower ranked—the monocotyledons. Further, the agaves and yuccas stand in nearly parallel divisions among monocotyledons—the agaves among the epigynous-flowered monocotyls, typified by the amaryllis family; the yuccas among the hypogynous-flowered congeners of the lily family. Both, moreover, are highly specialized representatives of their respective alliances, and of the two the agaves represent the higher character of development. Thus augmented interest is joined to all three when an outline of their position in the vegetable kingdom shows us that they are to be regarded as almost, if not quite, the highest products of the evolution of that ancient Aztec-American flora whose descendants they are.And so we are brought to realize that they were worthy recipients of all the attention Engelmann and his co-workers bestowed; and the history of their investigations becomes almost as interesting as the plants themselves. Foremost of all, as has been said, stand the labors of Engelmann; but with him are associated the names of many untiring explorers and enthusiastic botanists, each of whom contributed some vital element to the general outcome: Wislizenus, Emory, Torrey, Parry, Schott, Palmer, Newberry—all were workers in the field, and their names have gone down in the annals of botany appended to one species or another of the genera they studied, fittingly commemorating the aid they gave toward awakening scientific interest in this Southwestern plant group. Engelmann gathered together the work of all and compiled it in his masterly monographs, taking up first the cacti, then the yuccas, and finally the agaves. From time to time he published additional notes, as new store of information came to him, presenting most of the matter to the St. Louis Academy of Sciences, of which he was for years the leading support. Up to the month he died he was working over the great mass of notes he had accumulated on the cacti, preparatory to publishing a grand revision of his first monograph. That the work could not be completed is a source of deepest regret to living botanists; but, nevertheless, the original monograph still stands, and will continue to stand, as the backbone of our knowledge of the family it treats. And as to the other two monographs, the past decade has been able to add little to them of vital importance save in so far as more extended observations have served to more fully develop Engelmann's views. With justice, therefore, is Engelmann accorded a prominent place among scientists; but inseparably linked with his is the name of another man, honored as a broad-spirited patron of science, Henry Shaw, the founder of the Missouri Botanical Garden, on the outskirts of St. Louis. This was the pride of Engelmann's heart, and it was here that he constantly labored under the liberal patronage and never-failing encouragement of Shaw. The two men worked and planned together in their common interest, and as a result we find in the Missouri Garden to-day species of cacti numbering in the hundreds, of agaves more than half a hundred, and the better
part of all the known yuccas, altogether forming one of the most complete collections in the world of these Southwestern types; and he who carefully examines it will be ready to acknowledge it one of the most fascinating of all plant collections. Surely it could have no more fitting home than there in the city of Engelmann; and we can not but cherish the hope that no pains will be spared to make the collection even far more complete than it is, and thus give the American botanist a still greater laboratory in which to investigate so great a factor in American plant life. A suggestion of this aspect of the Missouri Garden may be found in the illustrations accompanying the present paper, most of which are from photographs taken there in July, 1892. The magnificent collection of cacti and the several flowering plants of agave in the World's Fair are of the highest interest in this connection. Can not these form a nucleus for a great permanent cactus garden?
From the general discussion we may appropriately pass to a more detailed sketch of each of the three groups before us, and in taking them up it will be found most convenient to place them in the order of their evolutionary rank: the cacti first, as representing the higher class of flowering plants; then the agaves; and lastly the yuccas, as somewhat lower in station than the second. This will have the merit, in addition to its logical virtue, of disposing of the weightiest group first, and of leaving till the last an amazing little entomological-botanical romance which gathers round the yucca. And the stately agaves will be not inharmoniously sandwiched between their two odd brethren. But let this suffice for a prospectus; the story will tell itself more satisfactorily.
Viewing the three members of our group together, the query presents itself: Is there not some vital significance in the relative extent and diversity of development in these three joint monarchs of the desert? Two of them, those we shall consider later, reach only the magnitude of genera, each constituting a moderate-sized and not remarkably diversified genus; while on the other hand the cacti together form an immense family, the natural order Cactceæ, aggregating over a thousand species, gathered into a number of genera. It is but a grand example of evolutionary principles, "natural selection and the survival of the fittest," for the facts must be interpreted in the light of Darwin's immortal phrase. The yucca pushes its sturdy rootstock through the sand and drinks up each available drop of water; the agave's succulent leaves store up a wealth of nutritious sap; but the cactus seems to be pre-eminently an invulnerable storehouse of life-giving moisture, and the veritable offspring of the arid, rocky sand-wastes, while the others seem only adopted children. Mark the peculiar characters of the typical cactus: The compact mass of the vegetative body, devoid of leaves or leaf-like appendages, exposes the least possible evaporating surface to the sun; the thick skin, bearing only a few scattered stomata, breathing pores, wraps an almost impermeable covering round the internal moisture; the long, wiry, fibrous roots run hither and thither through the sand and into the finest crevices of rocks; and further, the bristling, spiny armor shields the plant from men and beasts. All this and much more goes to make up the plant of plants that is best fitted to fulfill the mission of vegetation in the Southwestern borderlands. And so the problem solves itself, and we come to realize why the progenitors of the Cactaceæ, spread out and multiplied far and wide, and broke into a myriad varied forms, all retaining amid their diversity the distinctive characteristics of the primal type.
A standard British encyclopædia of scarcely three quarters of a century ago vouchsafes the statement that the order Cactaceæ embraces "twenty-seven" species. Grasp the contrast between this day of science and that! The botanists of the Century Dictionary estimate the species at far above the thousand mark. For this vast stride we thank a host of indefatigable explorers, and of these, a large share of credit goes to the scientists of the Mexican Boundary and the Pacific Railway Surveys, whose discoveries were the main foundations of the labors of Engelmann. Thus duly recognizing the scope of the field before us, we may with interest follow an outline sketch of the order. The general character of the vegetative body may be passed over for the present without further detail, that we may the more particularly notice the floral structure on which the systematic study of the order so largely depends. The inferior ovary, surmounted by the sepals, petals, and stamens, places the order immediately among the highest of the dicotyledonous Choripetalæ. There is a natural division into two suborders. In the first, the Rotatæ, the rotate many-leaved calyx and corolla, with the stamens, directly surmount the ovary. In the second there is a unique and obviously progressive development: calyx and corolla are united toward their bases and prolonged into a tube, with the stamens inserted on its throat—whence the name of the suborder, the Tubulosæ. The typical Rotatæ are the widespread genus Opuntia, numbering about two hundred and fifty species. The greater part of the genus are characterized by broad, thickened, fleshy, jointed stems; but a small subgenus, and this probably the less highly specialized, have cylindrical joints. Most of the species are more or less spreading and prostrate, but a large number are truly arborescent in growth. Several species are thoroughly naturalized in the northeastern quarter of the United States, and these are among the most brilliant acquisitions of our Northern flora. The gorgeous yellow of Opuntia Rafinesquii and O. Missouriensis, flourishing on a sandy hillside beneath the July sun, can well inspire the soul of botanist and flower-lover.
It is an interesting and significant fact that among the Rotatæ, as allies of the Opuntia, we find the two little genera that nearest approach being missing links between other neighboring orders and the Cactaceæ, otherwise almost utterly isolated. One is the genus Pereskia, in which, especially in Pereskia grandifolia, there are developed true leaves, succulent and veiny and with spines in their axils. Most species of the genus are shrubs or trees, and, still further remarkable, the flowers are borne in nearly panicled clusters. The thirteen species belong mostly to the West India region, and one produces the so-called "Barbados gooseberry" A decided analogy may be recognized by close comparison between these Pereskias and the Ribeseaceæ, the group of the saxifrage family which includes the currants and spiny-stemmed gooseberries; and this probably points to a distant connection between the progenitors of the Cactaceæ and those of the modern genus Ribes. The other possible "missing link" is the genus Rhipsalis, a curious group, mostly epiphytic, and growing in long, pendent masses from the branches of trees, in some instances resembling mistletoe. In these plants we see a possible approach to the group of so-called "ice-plants," the order Mesembryanthemaceæ. But most peculiar is the fact that one species of Rhipsalis is indigenous to South Africa, Madagascar, and Ceylon—the only instance of an Old World cactus. This probably has its significance.
The other suborder, the Tubulosæ, are undoubtedly the more highly specialized cacti, and further, significant fact, they are for the most part Mexican, while the Opuntiæ are widely scattered northward. Besides several minor genera of Tubulosæ there are three great and distinctive ones, which, as it is interesting to note, mark successive steps in structural specialization—they are Mamillaria, Echinocactus, and Cereus. In mamillaria there is a great departure, in the character of the vegetative body, from the Opuntæ. The plants are more or less globular or subcylindrical, and the original joints of the stem are indicated only by the conical spine-tipped tubercles which make up the surface of the fleshy mass. Echinocactus, the "hedgehog cacti," has the general appearance of mamillaria, but the tubercled surface is modified into a mere series of parallel vertical ribs, bearing clusters of spines along their ridges at points corresponding to the tubercles of mamillaria. Of the two genera, echinocactus is much the more strictly Mexican, while mamillaria has a few representatives spreading northeastward into Kansas and South Dakota. The large genus Cereus is the crowning glory of the cacti. It retains the ribbed structure of echinocactus, but its stems are nearly always columnar and in many instances arborescent. With echinocactus, this genus reaches its greatest development in Mexico, or near the boundary line, where flourishes the monarch of the Cactaceæ, the "giant cactus," Cereus giganteus. In cereus. furthermore, is the fullest development of the tubular floral structure, much less evident in echinocactus and mamillaria. Thus we find the cacti forming a little kingdom of their own, and could we here go beyond the limits of an outline, a broadly interesting study might be found in each division of the order. But without further detail the vitally important observation may be made, that there seems just reason to believe that what the Compositæ are to modern plant life in general, the Cactaceæ were, and
probably are, to that ancient Southwestern flora—the climax of its evolution.
Linnæus vividly expressed the spirit of the "century plant" in one Greek word, the very name he gave it—"Agave," so called, he said, "because that word indicates something grand and deserving admiration"; and although he only knew a half-dozen species, the many that subsequent research has brought to light justify most fully the title he bestowed. The genus holds a station of its own in the foremost ranks of monocotyledons, but, like the cacti among dicotyledons, rather isolated. It certainly approaches the amaryllis family in many characteristics, and, if really coming within the limits of this order at all, may perhaps be considered a highly specialized offshoot. There is immense variation in the foliage characters of agave, from the slender
reed-like leaves of Agave geminifolia to the massive blades of Agave Americana. The character of the inflorescence has been made the basis for the primary division of the genus into three groups: the Singulifloræ, the Geminifloræ, and the Paniculatæ. In all cases the flower-scape rises from the apex of the main axis of the plant; all the vital energy of often many years' growth is centered there, and the plant throws up its blossom-stalk as the supreme effort of its existence, and, when the fruit has ripened, dies—a strange phenomenon, and almost without parallel in any other so extensive group. In the Singulifloræ, is the simplest type of inflorescence. The flowers are loosely spiked, each one in the axil of a bract. To this group belongs our one Northern agave, the little Agave Virginica, which grows from Maryland and southern Indiana southwestward into Texas. The Geminifloræ have the flowers borne in pairs, and densely spiked along the scape. Variations which show transition between both these simpler groups and the third occur. The Paniculatæ have the scape more or less branching, often in the fashion of a candelabrum, each branch terminating in a dense cluster of flowers. These are the typical agaves, the crowning glory of the genus. The familiar Agave Americana is a representative of the Paniculatæ, and so also the plant shown in the accompanying photograph (Fig. 7), Agave Salmiana, a magnificent species that blossomed in the Missouri Botanical Garden in the summer of 1892. A splendid agave that commemorates the founder of the Missouri Garden is the Agave Shawii, dedicated by Engelmann to Henry Shaw; and in turn the labors of Engelmann have been fitly honored in the dedication to him of a most striking type that he once presented to the Missouri Garden. It blossomed there in the summer of 1891, and when it had been clearly proved a new species it was duly christened by Director Trelease Agave Engelmanni (Fig. 5). The structure of the agave flower is extremely unique in several particulars, but further detail can not be entered into. Almost every step taken in the investigation of the genus gives additional emphasis to the first impression, that it is one of the master marvels of plant life.
It remains to add some passing notes on the wonderfully beautiful genus which the lily family contributes to our group, the yuccas. A glorious floral offering to the arid Southwest highlands they certainly are, and scientifically their structure is in many ways scarcely less remarkable than that of the cacti and agaves. But the consideration of these points will be passed over here in order to call up more particularly the phenomenon that makes the yucca an astounding mystery to naturalist and philosopher, the manner of its cross-fertilization. For the fact is, we have here an extensive genus entirely incapable, save under most rarely extraordinary circumstances, of self-fertilization, and entirely dependent on one moth that fertilizes the flowers in order to insure food supply for its larvæ in the ripening seeds. The problem of yucca fertilization was for many years a vexed question; Engelmann spent much thought and observation in trying to learn its secret, and finally, over twenty years ago, called the attention of Prof. C. V. Riley, the noted entomologist, to certain moths which frequented the yucca flowers. After long and patient study and various erroneous speculations the two scientists ultimately brought the whole mystery to light; and in a recent paper, published in the Report of the Missouri Garden for 1892, Riley has fully elaborated his work of the earlier years and the observations made in the intervening time. The structure of the yucca flower
is plainly outlined in Fig. 10. Long experimentation has positively shown that it is practically impossible for the sticky pollen to be transferred from the little anthers on the tips of the short stamens to the fine stigmatic tube opening only at the tip of the pistil, except by external voluntary agency. As a matter of fact, the agency is always the little moths of the genus Pronuba—never any other insect whatsoever. Several different species of Pronuba frequent respectively several different species or groups of species of yuccas, but the most familiar one is the Pronuba yuccasella, always found on our Yucca filamentosa, the most Northeastern type. Just about
nightfall, as the flowers open, the moths are seen flitting about the yucca panicles. Usually the male is most constantly on the wing, while the female is found running about within the flowers. She begins operations by mounting the top of a stamen, exactly as shown in Fig. 11. There she scrapes with her two odd hooklike maxillary palpi the pollen out of the anther, and rolls It into a globular mass under her head. With this load, often thrice the size of her head, she goes to another flower, runs about, apparently examines every nook and corner of it, and then, if perchance satisfied, finally settles astride two of the stamens with her abdomen pressed down between them. From the tip of the abdomen the long, sharp-edged ovipositor, an organ of wonderful delicacy and most remarkable structure, is thrust into the tissue of the pistil, and the eggs are deposited among the ovules. This act may occur several times on the same pistil. Then, still more remarkable, the moth deliberately runs to the apex of the pistil and with tongue and palpi crams a portion of the collected pollen mass into the stigmatic tube, thereby fertilizing the flower. The tongue is worked up and down for some time in the tube like a piston rod, with evident intentness on the part of the moth. This
|Fig. 12.—Fruit of Yucca gloriosa.|
series of operations, always in the same order, may be repeated again and again till late into the evening. The moth chooses only the freshly opened flowers and those that have not been punctured by another moth coming before. Only the flowers thus fertilized can ever by any possibility produce fruit; and thus the yucca fruit, as seen in Fig. 12, always bears several constrictions where the scar was made by the puncture of the moth's ovipositor. Inside the fruit the moth-larva develops with the seeds, devouring sometimes a dozen of them; and when the pod ripens the larva eats its way out, and, in the night-time, drops to the ground by a silken thread, burrows into the soil, and there wraps itself in a strong cocoon. Sometimes the moth does not issue from the cocoon for several years, though not so generally. It is strangely true, however, that moth and yucca flower mature at the selfsame season; and strangest of all, it can not be shown that the moth during its short existence in the perfect state takes a particle of nutriment from the yucca flower—no pollen, no nectar, no stigmatic fluid. This is practically certain. Here, then, is a case of "instinct" that is utterly dazzling; it bids defiance to comparative psychology, philosophy, metaphysics—everything. Here is a plant that can not perpetuate itself without one certain strangely specialized moth; and here is a moth that can not live without that plant; and that moth deliberately cross-fertilizes the flowers without receiving any nutriment from them! Verily, the botanist must rise up and say of the yucca moth what Cicero said of the aged planter of orchards, "Serit arbores quæ alteri sæclo prosint!" and even with greater truth. This is interdependence between the worlds of insect and plant life that baffles understanding.
Cacti, agaves, yuccas, such is the three-typed group that stands out as a great division of a flora distinctively American; unique in the phase of plant development it presents; peculiar to a region of strange physical aspect; sprung directly, for aught that is known otherwise, from the mystery-shrouded soil of the Aztec and the cliff-dweller. And this is the splendid tribute the land of the far Southwest gives to the world of science.
- Figs. 5, 6, 10, 11, and 12 are from the Report of the Missouri Botanical Garden for 1892, and Fig. 8 is from the same report for 1893.
- In the Report for 1893 of the Missouri Botanical Garden, Prof. William Trelease, Director of the Garden, publishes a paper on yuccas, from which the following notes are made with reference to the Pronubas frequenting different species of the plant: Pronuba yucasella pollinates Y. filamentosa, Y. aloifolia, Y. glauca, Y. baccata, Y. gloriosa, Y. elata, Y. glauca, var. siricta; Pronuba synthetica pollinates Y. brevifolia; Pronuba maculata, Y. Whipplei; and Pronuba maculata, var. aterrima (a new variety of Pronuba discovered' by Prof. Trelease in 1892), pollinates Y. Whipplei, var. graminifolia; and, finally. Prof. Riley predicts the discovery of distinct species of Pronuba on each of these yuccas, viz.: Y. tillfera, F. treculeara, Y. Guatemalensis, and others.—H. L. C.