_ The Life of the Plant. 197 
life, vz. that organised structures are capable of a continual internal 
change ; and that so long as they are in contact with water and with 
atmospheric air, only a portion of their forces can be in equilibrium in 
their interior; in short, that they are in continual internal movement. 
The whole organism forms, therefore, a framework, between and in the 
molecules of which forces are constantly being set free by chemical 
changes, which forces again occasion further changes. ‘This is essen- 
tially dependent on the peculiar molecular structure which allows that at 
every point in the interior substances penetrate from without and are 
absorbed in the liquid or gaseous state, and can again be expelled. This 
internal changeableness attains its highest grade in the chlorophyll-bodies 
and the protoplasm. In the former, chemical changes take place with 
great energy and force under the influence of light, such as the formation 
of the green colouring matter and of starch ; while the absence of light 
at once sets up other chemical processes, which terminate only with the 
complete destruction of the whole of the chlorophyll-body. 
Besides the chemical and physical forces, others are also set free in 
plants by the mutual action on one another of their cells, which are 
manifested in a ¢emszon of the tissues or of the cells with respect to one 
another. 3 
Every part of the plant consists of external and internal layers whose 
growth proceeds, at least for a time, with a different rapidity in the 
same direction. A necessary consequence of this unequal energy of 
growth is a corresponding tension of the different layers with respect to 
one another ; because those whichare growing more quickly are prevented 
from expanding as much as their growth requires, and hence exercise a 
pressure on the layers that grow more slowly, which these seek to 
counteract by their elasticity. Such tensions of the layers caused by 
unequal growth may still continue after growth has ceased ; but they may 
also be destroyed by a change in the relative rates of growth. The © 
existence and the nature of the tension can be easily shown by sepa- 
rating the layers from one another; those which had grown more 
quickly, and were therefore previously compressed, will expand ; those 
which had grown more slowly and which were therefore previously 
stretched to more than their natural length, will now contract elastically ; 
the former become longer, the latter shorter, than before their separa- 
tion. A partial separation of the tissues is often sufficient to show 
this. If, for example, any rapidly growing stem is cut longitudinally 
into four pieces cross-wise, the pieces curve concavely outwards and 
convexly on their inner side, in consequence of the internal layers ex- 
tending while the outer layers contract. These tensions caused by the 
unequal growth in length of different layers of tissue act chiefly in the 
