a Pa ’ + ae Ld a ¢ | ae ht) ¥ wi ‘ - i “ oh 
wy  -A Sa ah eee ~ eh bia f, 4 ] 2 ales x ‘ 
~ = ¢ ie \ * ov vl . ; - 
ae » ; sat 
: ? ; ; / 
v- 4 ad . . wad 
| The Life of the Plant. ~ 215 
Since gravitation acts uninterruptedly on every part of the plant, 
all the parts must be so contrived as to diminish the influence of this 
force ; and we find, in fact, in the organisation of the plant, a number 
of contrivances for this purpose. ‘The firmness and elasticity of the 
wood of erect stems ; the uniform distribution on all sides of the weight 
of the foliage and of the branches; climbing and twining internodes 
which are sensitive to contact, and the various other contrivances that 
enable slender stems to climb which could not otherwise maintain 
themselves in an erect position under the weight of their leaves ; the 
floating apparatus of floating plants, such as the air-bladders in. the © 
fronds of Hucws and in the leaf-stalk of Zvapa, and those of Utricularia 
(Fig. 356, p. 185), the floating apparatus, such as the pappus or wings » 
of many seeds and fruits ; and a number of other specialities of organisa- 
tion, all serve to counteract the weight of the plant, and to prevent its 
sinking to the ground where this would be unfavourable to the other 
functions of the plant. Under the head of the tension of tissues it has _ 
alrcady been mentioned that young and weak plants in which there is — 
no tension tend to grow downwards in accordance with the force of 
gravitation ; while those in which there is tension have a tendency to 
grow erect; and this is the reason why roots in general grow down- 
wards and stems upwards. But the influence of gravitation on plants. 
is not yet exhausted, since it opposes a resistance to all internal move- 
ments, such as those of water and of the cell-sap whenever their direction 
is upwards, which resistance the plant must overcome by its internal 
forces. Light and gravitation are the forces which give to the separate 
parts of plants, such as the stem and leaves, their normal position, 
whether erect or horizontal, and therefore, along with the arrange- 
ments of the buds and leaves, impart to plants what is termed their 
Habit. 
From. the infinite variety of the conditions of life, it is 
evident that plants must possess a certain power of adapta- 
tion to their external conditions. 
This is seen especially clearly in amphibious plants, which live 
sometimes in water, sometimes on dry ground ; and in climbing plants. 
Thus for example, the terrestrial form of Polygonum amphibium, with 
its erect stem and shortly-stalked narrow hispid leaves, is easily trans- 
formed, by simply transplanting into water, into the floating form with 
floating long-stalked glabrous leaves. The branches of the terrestrial 
form then soon cease to grow, their leaves dry up, and the new plant 
of the floating form is developed from the rhizome. 
