Power to Adjust Parts to Surroundings 241 



conditions, its one-sided abundance is a matter of indifference 

 to them. This fact illustrates anew the adaptive character of 

 these responses; for it is a general rule that plant parts are in- 

 different to stimuli to which there is no profit in responding. 



The hydrotropism of roots involves matters of some practical 

 consequence. It is said that when trees develop in a uniform 

 soil, the root tips tend to collect in a circle just under the outer 

 drip of the foliage, which is obviously the place where the water 

 is usually most plenty. But in case the soil is moister on one 

 side than another, the roots grow more freely in that direction, 

 and may even extend to a distance several times the diameter of 

 the tree. In their progress thus towards the most copious wet- 

 ness, they sometimes are led to a drain, and, insinuating them- 

 selves through some crevice left in the tiles, find therein a com- 

 bination of water, air, and mineral substances so agreeable that 

 they grow very profusely, even to so great a degree that they 

 sometimes choke the drain quite completely. 



Chemotropism. — But roots have also other irritable responses, 

 notably to some chemical substances. Thus they turn, though 

 rather feebly, towards a source of supply of some of the minerals 

 they absorb, and this is typical chemotropism, with a very ob- 

 vious advantage. But they turn much more strongly towards 

 air (a special phase of chemotropism called aerotropism), — of 

 course for the oxygen it contains, which they need for their 

 respiration. It is easy to see in these cases how the stimulus is 

 received by the root, for the chemical substance, especially the 

 oxygen, must react with some of the materials found in the 

 complicated protoplasm with which it first comes into contact, 

 thus originating a differential chemical disturbance wliich would 

 establish the line of direction. 



But other structures besides roots are markedly chemotropic. 

 Thus pollen-tubes in their growth turn towards the substances 

 secreted by stigmas and styles. In the fertilization of Ferns, an 

 egg-cell at the bottom of a protective flask-like archegonium is 



