An introduction to physiological and systematical botany/Chapter 21
ON THE DISEASES OF PLANTS, PARTICULARLY AS ILLUSTRATIVE OF THEIR VITAL PRINCIPLE.
The diseases of Vegetables serve in many instances to prove their vitality, and to illustrate the nature of their constitution.
Plants are subject to Gangrene or Sphacelus, especially the more succulent kinds, of which a very curious account, concerning the Cactus coccinellifer, Indian Fig, or Nopal, extremely to our present purpose, is given by M. Thiery de Menonville, in his work on the culture of the Nopal as the food of the Cochineal insect. This writer travelled, about 20 years since, through the Spanish settlements in South America, chiefly noted for the cultivation of this precious insect, on purpose to transport it clandestinely to some of the French islands. Such were the supineness and ignorance of the Spaniards, that he succeeded in conveying, not only the living insects, but the bulky plant necessary for their sustenance, notwithstanding severe edicts to the contrary. He had attended previously to the management of the Nopal, and made his remarks on the diseases to which it is liable. Of these the Gangrene is extremely frequent in the true Nopal of Mexico, beginning by a black spot, which spreads till the whole leaf or branch rots off, or the shrub dies. But the same kind of plant is often affected with a much more serious disease, called by Thiery "la dissolution." This seems to be a sudden decay of the vital principle, like that produced in animals by lightning or strong electricity. In an hour's time, from some unknown cause, a joint, a whole branch, or sometimes an entire plant of the Nopal, changes from apparent health to a state of putrefaction or dissolution. One minute its surface is verdant and shining; the next it turns yellow, and all its brilliancy is gone. On cutting into its substance, the inside is found to have lost all cohesion, being quite rotten. The only remedy in this case is speedy amputation below the diseased part. Sometimes the force of the vital principle makes a stand, as it were, against the encroaching disease, and throws off the infected joint or branch. Such is the account given by Thiery, which evinces a power in vegetables precisely adequate to that of the animal constitution, by which an injured or diseased part is, by an effort of Nature, thrown off to preserve the rest.
Nor need we travel to Mexico to find examples of this. Every deciduous tree or shrub exhibits the very same phænomenon; for the fall of their decaying foliage in autumn, leaving the branches and young buds vigorous and healthy, can be explained in no other way. Yet Du Hamel laboured in vain to account for the fall of the leaf; nor is it wonderful that he or any body else, who endeavours to explain the physiology of vegetables or of animals according to one principle only, whether it be mechanical or chemical, should entirely fail. To consider the fall of leaves in autumn as a sloughing, or casting off diseased or worn out parts, seems so simple and evident, as to be hardly worth insisting upon. Yet I find myself anticipated in this theory by one physiologist only, named Vrolick, cited by Willdenow, in his Principles of Botany, p. 304, though several learned speculations to no purpose are extant on the subject. It is but just, however, that I should relate what led me to consider the matter with any attention. My observing friend Mr. Fairbairn of Chelsea garden long ago remarked to me, that when he had occasion to transplant any tree or shrub whilst in leaf, he could soon judge of its success by the ease with which its leaves were detached. The consequence of such treatment is more or less injury to the health of the plant, as will first appear by the drooping of the leaves, most of which will probably die, and the decay will generally be extended to the younger more delicate twigs. The exact progress of this decay may speedily be known, by the leaves of those branches which are irrecoverably dying or dead, remaining firmly attached, so as not to be pulled off without a force sufficient to bring away the bark or buds along with them: whereas the leaves of parts that have received no material injury, and where the vital energy acts with due power, either fall off spontaneously, or are detached by the slightest touch. Plants of hot countries, kept in our stoves, exhibit the same phænomenon when transplanted or otherwise injured, even though not naturally deciduous.
So when fruits are thoroughly ripened, they become, with respect to the parent plant, dead substances, and, however strongly attached before, are then thrown off as extraneous bodies. Their stalks fade or wither, though the life of the adjoining branch continues unimpaired, and a line of separation is soon drawn. In a poor soil, or unfavourable climate, a bunch or spike which should naturally consist of a considerable number of flowers, bears perhaps not half so many. Its upper part very early withers, the vital principle ceases to act at the point beyond which it could not continue to act with effect, and all its energy is directed to perfect what lies within the compass of its resources. This is evident in Lathyrus odoratus, the Sweet Pea of our gardens, a native of a very hot climate, at the summits of whose flower-stalks are generally found the rudiments of one or more flowers, not attempted to be perfected. So also the first Barley sown on the sandy heaths of Norfolk, and indeed too many a following crop, bears very few grains in an ear; for the same meagre supply of nourishment, bestowed equally on a numerous spike of blossoms, would infallibly starve them all. In like manner one seed only is perfected in the best wild Arabian Coffee, known by its round form; while the West Indian plantation Coffee has two in each berry, both consequently flattened on one side. The former grows in barren open places, in situations sufficiently favourable for the impregnation of its blossoms, but far less so for the perfecting of much seed; while the latter, well supplied with manure and moisture, is enabled to bring every germ to maturity.
Very strange effects are often produced upon plants by the attacks of insects, whence the various kinds of Galls derive their origin. These are occasioned by the punctures of those little animals, chiefly of the Hymenoptera order, and of the genus Cynips, in some vigorous part of the plant, as the leaves, leaf-stalks, young stem or branches, and sometimes the calyx or germen. The parent insect deposits its egg there, which is soon hatched, and in consequence of the perpetual irritation occasioned by the young maggot, feeding on the juices of the plant, the part where it is lodged acquires a morbid degree of luxuriance, frequently swelling to an immoderate size, and assuming the most extraordinary and whimsical shapes. This often happens to the shrubby species of Hawkweed, Hieracium sabaudum, Engl. Bot. t. 349, and umbellatum, t. 1771, whose stems in consequence swell into oval knots. Several different kinds of Galls are borne by the Oak, as those light spongy bodies, as big as walnuts, vulgarly named Oak apples; a red juicy berry-like excrescence on its leaves; and the very astringent Galls brought from the Levant, for the purposes of dyeing and making ink, which last are produced by a different species of Quercus from either of our own. The common Dog-rose, t. 992, frequently bears large moss-like balls, in whose internal parts numerous maggots are always to be found, till they become the winged Cynips Rosæ, and eat their way out. Many of our Willows bear round excrescences, as large as peas, on their leaves; but I remember to have been very much astonished in Provence with a fine branched production on the Willows in winter, which appeared like a tufted Lichen, but proved on examination a real Gall. Indeed our Salix Helix, t. 1343, is called Rose Willow from its bearing no less remarkable an excrescence, like a rose, at the ends of some of its branches, in consequence of the puncture of an insect, and these are in like manner durable though the proper leaves fall. The Mastic-tree, Pistacia Lentiscus, is often laden, in the south of Europe, with large red hollow finger-like bodies, swarming internally with small insects, the Aphis Pistaciæ of Linnæus. The young shoots of Salvia pomifera, Fl. Græc. t.&mbsp;15, S. triloba, t. 17, and even S. officinalis, in consequence of the attacks probably of some Cynips, swell into large juicy balls, very like apples, and even crowned with rudiments of leaves resembling the calyx of that fruit. These are esteemed in the Levant for their aromatic and acid flavour, especially when prepared with sugar.
It may be remarked that all the excrescences above mentioned are generally more acid than the rest of the plant that bears them, and also greatly inclined to turn red. The acid they contain is partly acetous, but more of the astringent kind.
The diseases of the skin, to which many vegetables are subject, are less easily understood than the foregoing. Besides one kind of Honey-dew, already mentioned p. 189, something like leprosy may be observed in Tragopogon major, Jacq. Austr. t. 29, which, as I have been informed by an accurate observer, does not injure the seed, nor infect the progeny. The stem of Shepherd's Purse, Engl. Bot. t. 1485, is occasionally swelled, and a white cream-like crust, afterwards powdery, ensues. The White Garden Rose, Rosa alba, produces, in like manner, an orange-coloured powder. It proves very difficult, in many cases, to judge whether such appearances proceed from a primary disease in the plant, arising from unseasonable cold or wet, or are owing to the baneful stimulus of parasitical fungi irritating the vital principle, like the young progeny of insects as above related. Sir Joseph Banks has, with great care and sagacity, traced the progress of the Blight in Corn, Uredo frumenti, Sowerb. Fung. t. 140, and given a complete history of the minute fungus which causes that appearance. See Annals of Botany, v. 2. 51, t. 3, 4. Under the inspection of this eminent promoter of science, Mr. Francis Bauer has made microscopical drawings of many similar fungi infecting the herbage and seeds of several plants, but has decided that the black swelling of the seed of corn, called by the French Ergot, though not well distinguished from other appearances by the generality of our agricultural writers, is indubitably a morbid swelling of the seed, and not in any way connected with the growth of a fungus. The anthers of certain plants often exhibit a similar disease, swelling, and producing an inordinate quantity of dark purplish powder instead of true pollen, as happens in Silene inflata, Fl. Brit. Engl. Bot. t. 164, and the white Lychnis dioica, t. 1580, whose petals are, not uncommonly, stained all over with this powder. Our knowledge on all these subjects is yet in its infancy; but it is to be hoped, now the pursuit of agriculture and of philosophical botany begin to be, in some distinguished instances, united, such examples will be followed, and science directed to one of its best ends, that of improving useful arts. And here I cannot but mention the experiments continually going on under the inspection of the ingenious Mr. Knight, of fertilizing the germen of one species or variety with the pollen of another nearly akin, as in apples, garden peas, &c., by which, judiciously managed, the advantages of different kinds are combined. By the same means Linnæus obtained intermediate species or varieties of several plants; and if any thing were wanting to confirm his theory respecting the stamens and pistils, this alone would place it out of all uncertainty.
- See his Phys. des Arbres, v. 1. 127.