OCR Text |
Show 558 SUMl\1All Y AND CHAP. XII. 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 severo 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 sen· siti veness, for there is no reason to believe that they profit by moving when touched. Our seedling now throws up a stem bearing lea-res, and often branches, all of which whilst young are con· tinually circumnutating. If wo 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 tho power of a microscope) wo should see the tip of each rootlet endeavouring to sweep small ellipses or circles, as far as the pressure of the sur· rounding earth permitted. All this astouishing am~unt of movement has boon going on year after year smce the time when, as a seedling, the tree first emerged from the ground. Stems are sometimes developed into long runners or stolons. ~rhese circumn uta te 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. CIIAP. XII. CONCLUDING HEl\'IAllKS. 559 We have now to consider circumnutation in a modified form, as the source of several oTcat classc f r_rh d'fi . b s 0 movement. e mo I cation may be determined b in~ate cau~es, or by external agencies. Under the nr?t head we ~ee lea~c~ which, when first unfolded, stand in a vertiCal position,. awl gradually bend down wards as they grow older. W c see flower-peduncles bending down after ~he flower ha~ withe~·od,. and others rising up; or agnm, stems with theu tips at first bowed ~ownwards, so as to be hooke<l, afterwards straightenmg themselves ; and many other such cases. These changes of position, which are clue to epinasty or hyponasty, occur at certain periods of the life of tho plant, and are independent of any external agency. They are effected not by a continuous upward or do\mward 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 youno- circumnutate in the ordinary manner, but as soo~ 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, evidentlv to favour the stem catching hold of a support. Tl{e stem also circumnutates rather more equally to all sides than in the case of n~n-climbirig plants. This is conspicuously the case With those tendrils which consist of modified leaves, as these. sweep wide circles; whilst ordinary leaves usually mrcumnutate nearly in the same vertical plane. Flo.we~·peduncles when converted into tendrils have ~heu mrcumnutating movement in like manner greatly Increased. We now come to our second group of circumnu- |