Popular Science Monthly/Volume 26/March 1885/How Fungi Live in Winter
By BYRON D. HALSTED, Sc.D.
"HARD times" come to most living things. Plants as well as animals have periods when they need to conserve all their energies, husband all their vitality. All vegetation obeys the injunction to multiply and replenish the earth, but with the greatest determination when there are present suffering and impending death. A drought hastens the processes of reproduction, and insufficient nourishment encourages an early if not an abundant fruitfulness. In a climate where hot and cold, or wet and dry, seasons regularly succeed each other, many of our most common economic plants have adapted themselves to these stated changes of outward conditions, and run their course during a single growing season. Such plants constitute that large portion of our vegetation known as the annuals. The great sunflower, that grows into a giant in a single season and defies the summer sun and storm, falls an easy victim to the frosts of autumn. It, however, prepared the way for many successors, in the ripened seeds, each one of which when given favorable conditions will germinate, grow, reproduce its kind, and thus finish another cycle in the realm of vegetable life. The bean-plant, in a different way, climbs its appointed pole, enjoys the same sunshine and shower, produces its blossoms, fills long pods with ripened seeds, and gives up its life like all its fellows in the field. A corn-plant completes its growth in not far from a hundred days, and leaves its accumulated vitality stored up in the grains upon the ear. The prospective life and activity of a whole field of waving corn may be considered as stored up in a few pecks of apparently lifeless seed-corn safely housed in the granary. We thus see that in the annual plants the life of the species is, so to speak, carried over from one growing season to another in the ripened seed. The seed is also the form in which plant-life is easily transported from place to place. The seed of some hedgerow weed, as it becomes loosened from its attachment upon the lifeless mother-plant, and is blown for rods or even miles over the surface of the incrusted snow, is a familiar and perhaps striking example that may enforce the meaning to be here conveyed. The young plantlet in the seed, snugly packed within thick coats, is preserved from death, and at the same time is carried far from the place where it was produced. The seed is the offspring of the plant and the childhood of its kind, though so fashioned and protected that it can pass safely through a period of drought or cold when its parent would have succumbed. It is the motile or migratory state of plants, and many are the means of transportation by land, wind, wave, stream, passing herd, and flying bird, that are within its reach.
Biennial plants, like the beet, carrot, etc., spend one season in preparing for the coming days of inactivity and exposure, and close their careers the following year by using up the accumulated store of food in the roots, stems, or leaves, in producing a crop of seeds. These plants have taken one bold step toward that perennial condition of life enjoyed by our shrubs, trees, and many other plants. Even the "giants of the forest" prepare themselves for the trying months of winter; by withdrawing their vital fluid from the delicate leaves, and with apparently lifeless branches bearing buds enwrapped in scales and secured with a natural glue, they brave the winter blasts.
The season of growth is constantly anticipating the days when the streams of vitality must be checked. The gardener may remove his tender plants to a place "under glass," and so change the order of Nature that things get "out of season," but soon the tortured plants must have rest from their labors and an opportunity to reproduce their kind.
With this somewhat lengthy introduction to our subject let us enter a less familiar field of plant-life, and see if we do not find the same rule holding true among the minute and frequently very troublesome plants known as Fungi. Enough is not known of the habits of these low forms of vegetation for us to measure the natural limits of individual existence; in fact, individuality is very obscure, and, being largely creatures of circumstance, multiplication is extremely rapid when conditions are most favorable, and at a standstill when the reverse is true. Some of the simplest forms of this vast group, like the minute bacteria, yeast-cells, etc., pass through many generations in a few hours; while, on the other hand, the larger species of the hard, woody "shelf" fungi, on trunks of trees and old stumps, may represent in a single so-called "individual" the accumulated growths of a score or more of years. The mildews, molds, and fungi of that description arc popularly considered as very transient, and as frequently lasting for only a day. They start from a simple body called a spore, produce a network of fine threads, yield a crop of spores in a few hours, and the cycle of life is completed. We, however, find that this round of multiplication is varied, and even these evanescent mildews produce structures which serve the special purpose of carrying the species over trying times of starvation, drought, or cold. The common bread-mold (Mucor stolonifer), for example, so abundant upon stale viands in warm, moist weather, forms spore-bearing capsules upon the tips of perpendicular threads, which, when ripe, burst, and the multitudes of minute spores are scattered in all directions by every passing breeze. As the bread begins to get dry, and fails to yield a full supply of nourishment, the mold commences to develop a second form of spore. These are produced by the union of the contents of two filaments (conjugation), as shown in Fig. 1. The two Indian-club-shaped branches are touching, end to end, at a and b; the union is
Fig. 1.—Formation of Resting Spores or Bread-Mold.
more advanced at c, while at d a central cell has formed, containing the mingled contents of the two united cells. At e is seen the mature spore, which has a thick, hard, black, and spiny covering, well adapted to protect its highly vitalized contents from all injury. This spore, though magnified ninety times in the engraving, is very much larger than the exceedingly minute sacs of protoplasm formed in the capsules above mentioned. The latter germinate at once, when favorably situated, but the larger black spore remains dormant for a considerable time. Rapid reproduction is provided for in the multitudes of small summer spores, while the preservation of the species is the end sought in the formation of the thick-coated black spores.
Passing from the minute molds that thrive upon bread, cake, etc., we come next to those parasitic species of fungi growing in the tissues of higher forms of plants. The grape-mildew (Peronospora viticola) is a familiar destructive fungus pest that will serve our purpose in an attempt to show the preparation these low forms of plant-life make for passing over hard times, including the winter season. In early-summer the lower surface of affected grape-leaves is covered with a minute forest of white filaments. The threads of the fungus run in all directions through the tissue of the leaf, and, coming to the surface, pass through the breathing-pores of the epidermis, and afterward branch and bear oval spores on the tips of the filaments. Fig. 2 represents a small portion of a branch of the lettuce-mildew, which is in the same genus, and closely related to the mildew of the grape. The branch is magnified two hundred times, and the spore A and its attachment are enlarged five hundred diameters. The multitudes of spores borne upon the tips of the branches are for
the rapid propagation of the mildew. They germinate by producing zoöspores—that is, the contents divide into six or more oval bodies, which soon rupture the spore-wall and escape, each provided with two hair-like appendages termed cilia. These motile zoöspores reproduce the mildew in a new place upon the same or another grape-leaf. Later in the season, and within the substance of the leaf or fruit, a second form of spore is found. This is termed a sexual spore, and requires the union of the contents of two peronospora filaments for its production. One of the thread-tips becomes much enlarged, as shown in Fig. 3, o (on the left), which represents the female cell charged with granular protoplasm. Another thread, n, arises near by, becomes club-shaped, and applies itself to one side of the female sphere. The contents of the male cell enter and mingle with those of the female, after which the latter matures into a spore. The process of fertilization is further shown in the portion of the engraving on the right, all parts being magnified three hundred and fifty times. The dark central portion is the spore, the contents of which become securely protected by the thick, hard coverings. Unlike the small, thin-walled spores borne upon the tips of the aërial branches, these large sexual spores remain through the winter before germinating. The vitality of the grape-mildew is concentrated and protected, thus enabling the fungus to survive an exposure that might otherwise prove fatal. It should also be kept in mind that the substance of the leaf in which these spores are imbedded aids in shielding them from harm. If the vineyardist could destroy all these sexual spores at the end of the growing season, he would have little further trouble from the destructive mildew.
The members of the Cruciferæ, or cabbage family of plants, are quite generally attacked by white molds—so called because they cover the affected parts with a coating of almost pure white. A much magnified (four hundred times) view of the non-sexual spores is seen in Fig. 4. The white spores are produced in rows, forming below
Fig. 4.—Summer Spores of White Molds.
and falling away from the top, in the simplest possible manner. The contents of the spores divide in germination as in the grape-mildew above described. A ruptured spore is shown one thousand times magnified, in the lower right-hand corner of the figure, while the ciliated protoplasmic bodies (zoöspores) are seen above. These white molds are provided with sexual winter spores very similar to those described for the grape-mildew; in fact, the two groups are closely related, and belong to the same family of fungi. Fig. 5 shows the mature sexual spores of Cystopus candidus, magnified four hundred diameters, one of which has its hard shell ruptured and its contents of zoospores escaping. It requires weeks and even months for the development of these sexual spores, and frequently a long time may elapse before germination takes place. On this account they have received the very appropriate name of resting-spores. They remain within the tissue of the plant, and are frequently liberated only by its decay. The life of the white molds passes over from one season to another in these rough, thick-coated spheres of protoplasm, the formation of which approaches in complexity that of the seeds of higher plants. The various rusts and brown mildews furnish very complicated methods of propagation, there being no less than four kinds of spores
produced before the whole life-history of some species is complete. Beginning with the cluster-cup form, found abundantly on the barberry-leaves, as shown in Fig. 6, it is known that the
spores from these "cups" produce the common rust upon the wheat leaves and stems. Later in the season another form of spore is formed in the same ruptured patches before occupied by the orange-rust spores. These last spores are double, and form slowly on the tips of slender filaments. Fig. 7 represents a cross-section through a pustule of brown mildew, two hundred times magnified, with the spores congregated beneath the ruptured epidermis. These dark patches and streaks remain until spring. When the spores germinate, as shown in Fig. 8, magnified five hundred times, each twin-spore sends out a filament that bears from three to five small oval bodies, known as sporidia. These will germinate on the barberry-leaf and develop the cluster-cups with which we started. It is seen that the rust has ample means for a rapid propagation of its kind during the growing season, and finally stores up its vitality in dark, thick-coated spores that remain on the stubble through the winter, and are not injured by sudden and severe changes in the weather.
It is due the reader, in passing, to state that one or more of the four forms of spores here briefly mentioned may be omitted. In warm climates it is possible for the winter spores to be dispensed with, the ordinary rust-spores being able to remain alive and continue the life of the pest. It is also believed that in regions unknown to the barberry the cluster-cup form may also be omitted. Like all other living things, the rusts accommodate themselves to circumstances, though watchful that their members do not decrease from any lack of vigilance on the part of these parasites.
The order Perisporiaceæ illustrates our subject still further. The members of this group of fungi are mainly parasites upon higher plants, forming a whitish, web-like film over the surface of the affected parts. In the early life of these white mildews, the horizontal threads send up vertical filaments in which partitions rapidly form at regular intervals. The cells thus produced are spores which fall away in succession from the top. A single vertical filament is shown at I, Fig. 9.
Fig. 9.—Spore Formation of White Mildews.
The spores thus produced are minute, the winds easily disperse them, and they quickly germinate, giving rise to new filaments of mildew. The formation of the sexual or winter spores begins late in the season and is shown in the remaining portions of the engraving. When these spores are to be formed, two filaments, crossing each other as shown at III, send out short projections. One of these, c, becomes the female part, and the other, b, the male portion. As a result of fertilization, eight or more branches, IV, h, grow up from the base and envelop the female cell. These branches continue to grow until a thick, hard covering is formed. This spore-case is shown more advanced in V. Within, one or more sacs containing spores are formed. At II is seen a mature spore-case which has been ruptured and a sac containing five spores has nearly escaped. It is seen at a glance that the provision made by these species of fungi for the protection of its spores during winter is most complete. Each spore is, doubtless, surcharged with vitality; around it is a sac, and outside of this a thick, hard covering not easily broken. Many of these spore-cases are provided with hooked appendages, by means of which they may hold fast to rough surfaces, and thus the contents are further protected.
The black-knot (Sphæeria morbosa), so injurious to the plum and cherry trees, furnishes an illustration of another family of fungi, many members of which are considered as perennial. The fungus attacks the young branches, causing them to swell to several times their natural size. The enlarged portion of the branch is made up of a vast number of minute threads, which increase in length and size until the bark of the twig is ruptured in one or more places. An olive-green covering soon forms over the exposed part, consisting of spores borne singly upon the tips of fungus-threads. These simple reproductive bodies quickly pass away and spread the disease to other parts. Later in the season the knot becomes incrusted and a second form of spore is produced, very different in form from the simple oval ones already mentioned. As autumn approaches, the knot assumes a black and rough appearance, indicated in Fig. 10. In the hard crust small pits are formed, in which spores are slowly produced within long, slender sacs. These spores are not ripe until toward spring. In this well-named black-knot we have a fungus with at least three forms of spores, one of which serves the important purpose of carrying the species through the winter season, in a form admitting of a ready dispersion in the early spring. The knots last for more than one season, thus showing that the whole community of fungus-life on a single plum or cherry branch is perennial. This is well shown when a gardener fails to cut away the branch for a sufficient distance below the affected part, in which case the remaining end will develop into a well-formed knot the following season. The filaments of the fungus extend for a foot or more below the swelling, and live on from year to year.
Belonging to the same great group of fungi with the black-knot, but furnishing a different illustration of our subject, is ergot (Claviceps purpurea). This fungus attacks the young female portion of the flower of many grasses, and replaces the grain with a hard, irregular mass, several times the size of the unaffected grain. From the resemblance of these grains to the spurs of a cock, and because they are most abundant upon the rye, the fungus has received the common name of "spurred rye." Fig. 11 shows a head of rye, natural size, with four of its grains ergoted. Multitudes of minute spores are produced upon the surface of the affected grain during the growing season. These, as the reader will naturally infer, are for the rapid propagation of the ergot elsewhere. Usually nothing further in spore-formation takes place until the following spring. The hard, purplish ergot-grains contain no spores, being simply indurated masses of threads containing a very large per cent of oil. This is the winter condition of the fungus. With the warmth of spring and the moisture of the soil upon which the grains may be lying, the horny spurs soften and send out stalks from one or more sides, which have club-shaped extremities bearing multitudes of long, slender spores in minute, pear-shaped sacs. Fig. 12 shows a "growing" ergot-grain in the upper left-hand corner; a cross-section of a head below; a more highly magnified view of a
pit on the right, and near the center of the engraving is a single spore-case with the needle-like spores protruding.
The ergot is not alone in assuming a hard condition for the purpose of getting over a severe period of either cold or drought. The term sclerotia, meaning hard, has long been employed as a name for the compact, resting condition common to many fungi. In the early history of this group of plants, Sclerotium was the name of a prominent genus abounding in many so-called species. Now, of course, this genus has passed away, because founded upon a single condition which any species may assume for self-preservation. The fine threads of various toadstools frequently become joined in long, hard masses, and may be found at the base of almost any decaying stump. It is not unreasonable to suppose that a fungus, otherwise short-lived, may survive the trying circumstances of heat and cold for a score or more of years in this hibernating state.
Fig. 12.—Ergoted Grains producing Spores.
The spawn of the common mushroom is a familiar example, to many, of the apparently lifeless condition which the threads of a fungus may assume. This spawn, consisting of the dried filaments of the mushroom, is sold in bricks, and afterward placed in beds which supply the proper heat, moisture, and nourishment for the growth of the edible mushrooms. Yeast, representing the smaller kinds of fungi, can also be kept in dried cakes, ready at an hour's notice to spring into activity and make our bread light and wholesome. It would be interesting to descend lower in the scale of plant organization (if it is lower), and see even among the bacteria, now brought so prominently before the world by the labors of Pasteur and Koch, that these minute organisms, after exhausting the nourishment from a liquid, form a precipitate, which may be regarded as a resting state. Enough, however, has been given to show that fungi, though a humble group of plants, do not lack for methods of rapid increase when favorable conditions prevail, and have abundant means for sustaining life during periods when growth is impossible.