With many dicotyledonous seedlings, as has lately been described by De Vries, the contraction of the parenchyma of the upper part of the radicle drags the hypocotyl downwards into the earth; sometimes (it is said) until even the cotyledons are buried. The hypocotyl itself of some species contracts in a like manner. It is believed that this burying process serves to protect the seedlings against the frosts of winter.
Our imaginary seedling is now mature as a seedling, for its hypocotyl is straight and its cotyledons are fully expanded. In this state the upper part of the hypocotyl and the cotyledons continue for some time to circumnutate, generally to a wide extent relatively to the size of the parts, and at a rapid rate. But seedlings profit by this power of movement only when it is modified, especially by the action of light and [page 558] gravitation; for they are thus enabled to move more rapidly and to a greater extent than can most mature plants. Seedlings are subjected to a severe struggle for life, and it appears to be highly important to them that they should adapt themselves as quickly and as perfectly as possible to their conditions. Hence also it is that they are so extremely sensitive to light and gravitation. The cotyledons of some few species are sensitive to a touch; but it is probable that this is only an indirect result of the foregoing kinds of sensitiveness, for there is no reason to believe that they profit by moving when touched.
Our seedling now throws up a stem bearing leaves, and often branches, all of which whilst young are continually circumnutating. If we look, for instance, at a great acacia tree, we may feel assured that every one of the innumerable growing shoots is constantly describing small ellipses; as is each petiole, sub-petiole, and leaflet. The latter, as well as ordinary leaves, generally move up and down in nearly the same vertical plane, so that they describe very narrow ellipses. The flower-peduncles are likewise continually circumnutating. If we could look beneath the ground, and our eyes had the power of a microscope, we should see the tip of each rootlet endeavouring to sweep small ellipses or circles, as far as the pressure of the surrounding earth permitted. All this astonishing amount of movement has been going on year after year since the time when, as a seedling, the tree first emerged from the ground.
Stems are sometimes developed into long runners or stolons. These circumnutate in a conspicuous manner, and are thus aided in passing between and over surrounding obstacles. But whether the circumnutating movement has been increased for this special purpose is doubtful. [page 559]
We have now to consider circumnutation in a modified form, as the source of several great classes of movement. The modification may be determined by innate causes, or by external agencies. Under the first head we see leaves which, when first unfolded, stand in a vertical position, and gradually bend downwards as they grow older. We see flower-peduncles bending down after the flower has withered, and others rising up; or again, stems with their tips at first bowed downwards, so as to be hooked, afterwards straightening themselves; and many other such cases. These changes of position, which are due to epinasty or hyponasty, occur at certain periods of the life of the plant, and are independent of any external agency. They are effected not by a continuous upward or downward movement, but by a succession of small ellipses, or by zigzag lines,--that is, by a circumnutating movement which is preponderant in some one direction.
Again, climbing plants whilst young circumnutate in the ordinary manner, but as soon as the stem has grown to a certain height, which is different for different species, it elongates rapidly, and now the amplitude of the circumnutating movement is immensely increased, evidently to favour the stem catching hold of a support. The stem also circumnutates rather more equally to all sides than in the case of non-climbing plants.