SENSITIVENESS 365 
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 gent 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 
organism. It is usually spoken of under the general term 
irritability or sensitiveness, 
