SENSITIVENESS 357 
They are now hereditary and to a certain extent indepen- 
dent of the changing conditions which the plants encounter. 
It may well be, however, that they have become impressed 
upon the organisation of vegetable protoplasm by the con- 
stant recurrence of these changes of the environment during 
the long ages of the past. This does not appear unlikely 
in the face of the fact that, as we shall see later, it is possible 
under appropriate conditions to impress a new rhythm 
upon particular organs. The manifestation of rhythmic 
change has, however, become one of the vital properties of 
protoplasm. 
We saw in an earlier chapter that the peculiarities of 
form and structure which different plants possess are to be 
associated with the character of their environment. From 
such facts as were there discussed it is evident that a plant 
is capable of receiving impressions from without and 
responding to them in various ways. If we examine any 
plant which does not show such marked adaptation to its 
surroundings as those which were then more particularly 
under consideration, we can still find evidence of the pos- 
session of a similar power of appreciating differences in the 
external conditions in which it finds itself, and of modifying 
certain of its vital processes In response. When certain 
zoospores of some of the lower Alge which swim freely in 
water are suddenly exposed to a brilliant light, they take 
up at once a definite position with regard to the incident 
rays. When a leaf of Mimosa pudica, the so-called sensi- 
tive plant, is roughly handled, it falls from its normal 
position and takes up a new one, while its leaflets become 
folded together. When a filament of Mesocarpus is exposed 
to an electric shock sent through the water in which it is 
floating, it is found not infrequently that it splits up into 
its constituent cells. This power of receiving impressions 
from without, to which we have had frequently to refer in 
discussing the phenomena of growth and rhythm, is another. 
property of vegetable protoplasm, and can be observed 
to belong, in a greater or less degree, to every vegetable 
