484 MANUAL OF BOTANY 



CHAPTER XIV. 



THE BELATION OF THE PLANT TO ITS ENVIRONMENT. 

 IRRITABILITY. 



We have seen in the last chapter that the peculiarities of 

 form and structure which different plants present are to be 

 associated with the character of their environment. From 

 such facts as were there discussed it is evident that the 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, we can 

 find evidence of the possession of a similar power of appreciating 

 differences in its external conditions, and of responding to them 

 in various vi'ays. Thus when certain zoospores of some of the 

 lower AlgsE, which swim freely in water, are suddenly exposed to 

 a brilliant hght, they at once take up a definite position with 

 regard to it. When a leaf of Mimosa jjudica, 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 Mesooarpus is exposed to an electric shock 

 sent through the water in which it is floating, it is found not in- 

 frequently that it sphts up into its constituent cells. This power 

 of receiving impressions from without is inherent in the proto- 

 plasm, and spoken of under the general term irritahility . 



Not only does it depend upon the protoplasm, but the latter 

 must be in a healthy condition to manifest it. When a dicotyle- 

 donous plant which has been growing under ordinary atmospheric 

 conditions, exposed to diffused daylight, is removed into darkness 

 and kept there for some time, it becomes incapable of thus 

 being impressed by its surroundmgs. Nor is its irritability alone 

 affected by the absence of light, for many of its parts, particu- 

 larly its leaves, cease to grow under such conditions. The 

 condition which is thus induced by light, and upon which the 

 manifestations of irritability depend, is known as Photoionus. 



Plants, then, when in a phototonic condition, have the power 

 to respond in various ways to alterations in their environment. 

 If we consider the nature of the environment in the case of 

 ordinary terrestrial plants, we find it usually as follows : the 

 root system is embedded in the soil, among the particles of 

 which the young root-branches ramify, and to them the root- 

 hairs become firmly attached ; the stem rises verticalh- into the 



