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Fig. 21.—Trifolium subterraneum fruit.
"When the flower-heads have reached the ground, the younger imperfect flowers in the centre are pressed together in the form of a cone, whereas the perfect, and the outer imperfect flowers are reflexed and closely surround the footstalk. Thus the form they assume is adapted to offer as little resistance as possible in penetrating the soil. The flower-heads are able to bury themselves in garden mould or sand. The depth to which they penetrate, measured from the surface to the back of the head, is from a quarter to half an inch. With plants in the house a head partly buried itself in six hours. After three days only the tips of the reflexed calices were visible. In six days the whole had disappeared. Plants growing out of doors are believed to bury their flower-heads in a much shorter time. Only a few of the flower-heads which from their position are not able to reach the ground yield seeds, whereas the buried ones never failed to produce as many seeds as there had been perfect flowers.
"It is unnecessary to enter into all the details of observations on the movement of the footstalk, from the time it begins to bend until the flower-head is buried in the ground. Suffice it to say, that throughout this period oscillation was going on. A peduncle was watched during fifty-one hours, whilst in the act of burying itself in a heap of sand. When buried so that the tips of the sepals alone were visible, it was rotating. When the flower-head had completely disappeared beneath the sand it was still rotating. Any one who will observe this process will be convinced that the rocking movement due to the rotation of the peduncle bears an important part in the act. Considering that the flower-heads are very light, that the peduncles are long, thin, and flexible, and that they arise from flexible branches, it is incredible that an object as blunt as one of these flower-heads could penetrate the ground by means of the growing force of the peduncle, unless it were aided by the rocking movement. After a flower-head has penetrated the ground to a small depth, another and efficient agency comes into play; the central rigid aborted flowers, each terminating in five long claws, curve up towards the peduncle, and in doing so can hardly fail to drag the head down to a greater depth, aided as this action is by the circumnutating movement, which continues after the flower-head has completely buried itself. The aborted flowers thus act something like the hands of a mole, which force the earth backwards and the body forwards."
Another instance, equally remarkable, is that of the "ground-nut," or "ground-pea" (Arachis hypogæa) which is cultivated in all tropical countries. After the flowers fall the stalk of the ovary elongates itself considerably, bends downwards, and the ovary is buried in the ground, where the pod is matured. It is said that flowers which grow too high on the plant to reach and bury themselves in the ground, do not produce seeds. The same phenomenon of rotation or oscillation is plainly visible in the ovaries directed towards the ground as in the previous example. Any sane person would arrive at the conclusion that this subterranean habit is of some service to the plant; that all this elaborate adaptation for burrowing is designed for some purpose, and is not merely accidental; and that the smaller contributories to the act, whilst they aid in its consummation, do so to that intent.
We have endeavoured to show, in this chapter, that the rotation, or oscillation, of the growing parts of plants is a phenomenon which is exceedingly common; that it exhibits itself first in the young radicle soon after it emerges from the seed; that it is very prevalent in the cotyledons or seed-leaves, and in the plumule or growing point; that the young parts of all plants evidence its presence to a greater or less extent; that leaves possess the faculty as well as cotyledons; and, finally, that not only do flower-stems oscillate, but that after the flowers have withered the same phenomenon accompanies the ovary of those plants which mature their seeds beneath the surface of the soil.
We have, by the way, illustrated the peculiarly sensitive character of the tip of the radicle, and endeavoured to indicate its service to the plant. It has only been our aim to summarise what is known of these phenomena, and to present the most striking features as subjects for thoughtful reflection, and as incentives to closer observation.
CHAPTER VIII
HELIOTROPES, OR SUNFLOWERS.
THE designation which we have adopted for this chapter is simply intended to intimate that we desire to include under it such observations as we purpose to make on plants which conspicuously turn themselves towards or from the sun, or parallel to its course,—
As the sunflower turns on its god as he sets The same look which it turn'd as he rose,
is somewhat amplified so as to include a greater variety of movements, more or less dependent on light. Thus, for instance, the American Compass-plant, which is affirmed to present the edges of its leaves duly north and south, although not truly heliotropal, could not be omitted. This plant has evidently been long familiar to the hunters of the prairies, on account of the direction of its leaves, although it was not made known to the scientific world until 1842.
Captain Mayne Reid calls it the Polar-plant, for he says of it, "We had a guide to our direction, unerring as the magnetic needle. We were traversing the region of the Polar-plant, the planes of whose leaves at almost every step pointed out our meridian. It grew upon our track and was crushed under the hoofs of our horses as we rode onwards." Under the same name it is referred to by Burton: "Whilst in the damper ground appeared the Polar-plant,—that prairie compass, the plane of whose leaf ever turns towards the magnetic meridian." The Times correspondent with the Prince of Wales in Canada alludes to it as the Compass-weed: "Fortunately none go to the prairies for the first time without being shown, in case of such mishaps, the groups of Compass-weed which abound all over the plains, and the broad flat leaves of which point due north and south with an accuracy as unvarying as that of the magnetic needle itself." Lieut. J. W. Albert says: "The prairie was yet what is called rolling, the flat bottoms were covered with the rosin weed, or Polar-plant (Silphium laciniatum), whose pinnate-parted leaves have their lobes extending like fingers on each side of the midrib. It is said that the planes of the leaves of this plant are coincident with the plane of the meridian; but those I have noticed must have been influenced by some local attraction that deranged their polarity." Another officer in the United States army calls it the "Pilot-weed." Hence it is evident that the belief is widely prevalent that its leaves affect polarity.
Look at this delicate plant that lifts its head from the meadow, See how its leaves all point to the north, as true as the magnet; It is the compass-flower, that the finger of God has suspended Here on its fragile stalk, to direct the traveller's journey Over the sea-like, pathless, limitless waste of the desert.
And, in confirmation of this description by the poet, it is stated, on authority, that "repeated observations upon the prairies, with measurements by the compass of the directions assumed by hundreds of leaves, especially of the radicle ones, have shown that as to prevalent position, the popular belief has a certain foundation in the fact." It has also been found that the anatomical structure of the leaves corresponds to this position. Since the leaves tend to assume a position in which the two faces are about equally illuminated by the sun, it has been observed that the stomata are about equally abundant on both sides, and the arrangement of "palisade cells" of both strata are nearly the same. Sir Joseph Hooker says, that when traversing the prairies with Professor Asa Gray, in 1877, he watched the position of the leaves of many hundred plants from the window of the railway car, and after some time persuaded himself that the younger, more erect leaves especially, had their faces parallel approximately to the meridian line. At the same time he says that he convinced himself that "the flower-heads of various of the great Helianthoid Compositæ, such as that which we call the 'sunflower,' follow the sun's motion in the heavens to a very appreciable degree, their morning and evening positions being reversed."
It has been truly said that no one can look at the plants growing on a bank, or on the borders of a thick wood, and doubt that the young stems and leaves place themselves so that the leaves may be well illuminated. Whoever has placed half a dozen plants on a window-sill knows well enough how soon all the leaves and extremities of the branches will be directed towards the window. The tendency of plants to turn to the light is known to every cultivator, but this tendency is more strongly developed in some plants than in others, and in some parts of the same plant than in others. The types of heliotropism, in its broadest sense, are four, and to these, for the sake of precision, distinct terms have been applied. In the first place, there are plants which when exposed to a strong lateral light turn speedily towards it. This is true heliotropism, or turning towards the sun. Then, secondly, there are plants which, when exposed in a similar manner to a bright side-light, manifestly and speedily turn from it. This has been called "negative heliotropism," but the term which Mr. Darwin has employed to designate such movements is apheliotropism. Thirdly, there are plants which on exposure to light, when sufficiently intense, place their leaves transversely to the direction whence the light proceeds, and this has been called "transverse heliotropism," or, as Mr. Darwin prefers to call it, diaheliotropism. And finally, there are those plants which direct their leaves by rising, or falling, or twisting, so that they may be less intensely illuminated, and these movements, which are sometimes called "diurnal sleep," the same author designates as paraheliotropism. It is clear that if grouped in accordance with their purposes the first and third of these types are allied, as also are the second and fourth. In "heliotropism" and "transverse heliotropism" the object is to turn into and take advantage of the light. In "negative heliotropism" and "diurnal sleep," on the contrary, the object seems to be turning from, or shunning as much as possible, the direct influence of the light. We have given above the four terms by which it is proposed to designate these four movements, but for our purposes we shall make as little use of them as possible.
True heliotropism prevails very extensively amongst the higher plants, but there are some remarkable exceptions; as, for instance, in the "carnivorous plants." The sundews and side-saddle flowers exhibit in their leaves and pitchers no trace of heliotropism. The stems, tendrils, and rootlets of climbing plants are often opposed to heliotropism, or negatively heliotropic, whilst the leaves, on the contrary, have a general tendency to turn towards the sun. Most seedlings are strongly heliotropic, even though afterwards, as they grow up, they become either uninfluenced by the direct light, or, in some cases, turn away from it. Evidently some of the lowest forms of vegetable life seek the light. Strasburger says that the cells of Haematococcus, a simple unicellular alga, moved to a light which only just sufficed to allow middle-sized type to be read. It is well known how the species of Oscillaria congregate towards the light. Some Desmids and Diatoms exhibit the same propensity, the latter especially will rise to the surface and form a scum on the water in the full blaze of sunlight.