Irritability 50 



as are brought about by the appKcation of an external force. 

 For example, a young fledgling falling out of its nest is mov- 

 ing in the same way as an apple falling off a tree or a leaf 

 fluttering in the breeze — that is, through the action of some 

 external force. A worm crawling upon the ground, how- 

 ever, or a gnat darting through the air, is thought of as mov- 

 ing because of some happening on the inside. This kind of 

 movement, in plants and in animals, is found by observation 

 and experiment to be due to contractions of the living sub- 

 stance. We can see extreme manifestations of this kind of 

 contraction in the work of the muscles or flesh of higher 

 animals. 



We do not ordinarily observe the movements of plant 

 structure because their movements are very slow, and es- 

 pecially because they take place on such a small scale that 

 we should not be able to see them even with careful watch- 

 ing, unless we used a microscope. There are many plant 

 movements, such as the expanding of a flower bud, or the 

 curling of a tendril, or the twisting of a seed pod, which are 

 observed without great difficulty. Such movements are in 

 large part due not to the contraction of the living substance 

 or protoplasm, but to uneven growth, or to uneven drying 

 or absorption of water. 



Movements of this last kind as well as other movements 

 are to be found in non-living substances, especially in artifi- 

 cial contrivances and in chemical substances which bring 

 about more or less violent movements in the course of their 

 changes. In the latter class are the various kinds of ex- 

 plosions which we can bring about by means of special 

 preparations. 



Irritability 



Another characteristic of living things is seen in our 

 own sensitiveness to what is going on around us. The prick 

 of a pin, a breath of cold air, the taste of sweet or sour, the 

 sound of passing traffic — these represent our own sensitive- 



