In the Liverworts (Hepaticae) the spores are mixed in the capsules with spiral threads, or elaters. If the contents of one of these capsules are moistened after they have become dry, the spiral threads will be seen wriggling and twisting about, by means of the relaxation of the spiral, such movement also being of assistance in the dispersal of the spores. In the same manner we have observed the threads in such Myxomycetes as Trichia, in which the threads are spiral, relax a little when moistened after they have become dry, but, in this instance, only to a limited extent.
The examples we have given are sufficient to show that, inasmuch as there are movements in plants which result from the influences of light, temperature, and other causes, as demonstrated in preceding chapters; so also there are movements which are dependent upon the slightest variation in the humidity of the surrounding atmosphere. The further we investigate the phenomena of plant-life, the more do we become assured that, if there is not "a soul in every leaf," there is at least a marvellous adaptation of the parts, like a well-ordered machine, in order to secure definite and essential results.
In all places, then, and in all seasons, Flowers expand their light and soul-like wings, Teaching us, by most persuasive reasons, How akin they are to human things.
And with child-like, credulous affection, We behold their tender buds expand: Emblems of our own great resurrection; Emblems of the bright and better land.
CHAPTER XIV
DISPERSION.
IN many cases there is undoubtedly a close relationship between the form which fruits or seeds assume, and the mode by which they are dispersed. If it be true, as some contend, that the ultimate object of every plant is its own perpetuation, then the dispersion of the seed is an important operation, which consummates all other acts, and it would be anticipated that adequate provision would be made to ensure its full attainment. This is certainly not accomplished by any uniform method, but through various agencies, and in a multitude of ways. We shall be able in some cases to comprehend distinctly how the operation is performed, whilst in others it is more complex, and sometimes obscure. One agency in dispersion is the wind, which wafts seeds that are provided with wings to their destination. Another agency is undoubtedly water in which they are floated to a congenial spot. Another is the application of local force, by elasticity or hygroscopy, by means of which the seeds are forcibly expelled. Another is specialised structure by aid of which various animals are utilised as a means of transport. In these, and various collateral ways, we are enabled to associate the modifications of form with the mode of distribution, and it is to a few of the most striking examples of these different modes that this chapter is devoted. Illustrations will also be found scattered through other portions of this volume incidentally, when given in association with other subjects, and especially in the chapter on "mimicry," in which the same type of structure, and, presumably, the same mode of dispersion will be found repeated in different and widely separated orders. Neither here nor there have we assumed the exhaustion of so fertile a theme.
When writing of hygroscopism we have already alluded to the facility with which the seeds are dispersed from such fruits as burst open with violence and jerk out their contents, as in the sand-box tree. In a similar manner we might have instanced Buettneria aspera, one of the order Sterculiaceae, but a more important tree in many respects is the Mahogany tree (Swietenia mahogani), the woody fruits of which separate so freely, and disperse the seeds, that it is difficult to meet with any but fragments of the capsule in the countries where it flourishes. Another advantage is possessed by this tree, in that the seeds themselves are winged, like a samara, so that two modes of dispersion are combined in the same fruit.
The balsams, which a few years ago were great favourites in country districts, and ornamented every cottager's window, scatter their seeds to a great distance by the violent rupture of the fruits, in allusion to which circumstance they have been called "Touch-me-not."
<!-- image -->Fig. 53.—Balsam (Impatiens).
A remarkable instance of violent dehiscence is to be seen in a member of the cucumber family, named from its habits the "squirting cucumber" (Momordica elaterium). The small oval fruit, not more than an inch and a half in length, is covered with prickles. When quite ripe this separates from the stalk, and jerks out its juice, with which the seeds are mixed, from the little opening at the base. The fruit was formerly considered valuable as a medicine, and was cultivated for that purpose in this country, but now that its use has been superseded it is rarely to be met with.
The witch hazel of North America (Hamamelis virginica) exhibits a peculiar elasticity in the seeds, or embryo of the seeds, which are thrown out with such force as to strike people violently in the face who pass through the woods. Collecting a number of the capsules, and laying them on the floor, Mr. Meehan found that the seeds, or embryos were thrown out, generally to the distance of four or six feet, and in one instance as much as twelve feet.
We might here descend a little lower in the vegetable world than we have done hitherto, and indicate amongst the lower cryptogamia one or two very decided instances of the forcible ejection of the mature fruit corpuscles. In fungi, for example, we have observed the grass all round a spot in which a number of cups of a "bird's nest fungus" (Cyathus striatus or Crucibulum vulgare) were growing, sprinkled to the height of six or eight inches with the ejected sporangia; and this is a species in which the sporangia are attached to the cup by an elastic cord, so that forcible ejection is not a recognised means by which distribution is affected. Yet no other explanation can be offered for the occurrence of the sporangia on the surrounding grass. Another species (Sphaerobolus stellatus) normally ruptures at the apex, and expels the globose sporangium, like a cannon ball, but no larger than a small pill into the air. Still more minute, and in another section, the little mucor-like fungus, which grows so profusely on cowdung, ejects the little sporangia to an enormous distance, in proportion to the size of the plant. We have seen them covering the leaves of vines, and other plants, in minute specks, like the dung of flies, at an almost incredible distance from their source. Although at first received with some doubt, C. B. Plowright has affirmed that the spores of some agarics do not simply fall from the hymenium, but are ejected, in some manner not yet explained, for three or four inches, not only in a line with, but above the plane from whence they proceed. We have since been able to corroborate the fact, in two or three instances, but without succeeding in tracing the cause, or being able to submit a reasonable theory to account for the phenomenon. Inasmuch as we were at first sceptical, with others, it is with greater pleasure that we recant.
Passing now to atmospheric dispersion, the seeds of composite plants are transported from place to place, chiefly by means of the pappus with which they are crowned. Although varying in different species, this coronet has but one purpose, the dispersion of the plant. "Elevated on the apex of a long beak, the parachute of the seed of the goatsbeard (Tragopogon pratensis) consists of a number of slender spokes, which diffuse themselves circularly, and are 'telarly interwoven,' somewhat after the fashion of the spiders web. This comparatively intricate structure is given as a countervail to the great size and weight of the seed. The down of the dandelion is supported on a long and slender pedicel, and is an object of vulgar admiration; but it scarcely equals in beauty the similarly patterned fruit of the helminthia. The thistle's-down is, on the contrary, sessile—the threads being sometimes only spinous, at other times plumed like a feather—and the down of the latter is peculiarly light. The coronet of the carline thistle is remarkable for its elegance and circular spread and plumage, and buoys easily its silky-coated seed. In the sow thistles what we most admire is the ribbed and striated seeds, but the down that diffuses them is abundant and of pure whiteness. The seeds of the coltsfoot afford an example of a structure, common in the order, where the seed is surmounted by a tuft of silken hairs armed, at regular intervals, with a series of denticles or spines, only visible with a good magnifier. We have a contrast to this in the curious fruit of the blue-bottle (Centaurea cyanea) which has a small tuft of asbestine spines at the base, and a large but short tuft of rigid stout lanceolate spines on the top, the edges of each of them indented with close and sharp serratures like a saw. This tuft cannot float the seed in the air, but it will obviously direct and hasten its descent into the soil, and it will be remarked that the forward direction of the spines must be opposed to every influence to cast them up again, after having been buried under the surface."
The stalks of the down in the dandelion contract closely together in moist and wet weather—a beautiful provision to secure its dispersion only in a dry day, when it is driven off by every zephyr, and not unoften by the schoolboy, who thus endeavours to resolve his doubts as to the hour:—
Dandelion, with globe of down, The school-boy's clock in every town, Which the truant puffs amain To conjure lost hours back again.
The dispersion of seeds by means of a coronet of delicate hairs is not confined to composite plants. A like provision may be observed in the willow herbs (Epilobium), and in many of the Apocynaceae, as well as the Asclepiadaceae. Yet this is only one provision for the dispersal of seeds by the agency of the wind. Another, and equally successful contrivance is the expansion of the sides of the seed into a membranous wing. These winged seeds reach their highest development in the trumpet flowers (Bignoniaceae), where the large wings extend three or four inches, and the seeds float like a large butterfly, wafted from place to place, until a secure home is reached. In our own country such winged seeds are usually minute, if we exclude the heavier and less delicate winged fruits of such trees as the maple, ash, and elm, which are called samarae. These latter are doubtless aided in their dispersion by means of their wing-like margins, to which we shall have occasion to refer again hereafter, when writing of the similarities which prevail in the organs of very diverse plants. We are justified, then, in asserting that special provision is made for the dispersion of many seeds through the air, by means of the wind.
Spiny fruits are found amongst the members of many of the families in the vegetable kingdom. It is evident that the rigid spines with which they are armed aid considerably in their dispersion. The name of "caltrops" has been applied to some of these, in allusion to the "calcitrapa" which was employed in ancient warfare to harass the enemy's cavalry. One of these kind of caltrops (Tribulus terrestris) is widely diffused, probably on account of the facility with which the fruits are transported in the wool of animals. They have an elegant, symmetrical, star-like form, and the spines are very sharp and rigid. Another, but less complex, fruit (Pedalium murex) has its dispersion provided for in a similar manner. The name of caltrops has also been given to one of our indigenous plants, called also "star-thistle," on account of the sharp spines of its woody involucre (Centaurea calcitrapa).