1 HI. AMKRITAN BOTANIST 161 



wliicli ht'iids upward through apog'eotropisui exerts consider- 

 able force, such as that of its own weight, wiiich has tn he 

 Hfted. This is often sufificient to cause the part to bend down- 

 ward; hut tlie downward course is rendered more obhcjuc l)y 

 the simultaneous circumnutating movement. 



HYDROTROPISM 



There is in roots an inherent tendency to move towards 

 a direction where moisture i> mo>t plentiful. The advantage 

 of hydrotropism is perfectly e\ident. for it is the very first 

 function of roots to absorb water. It is believed that water 

 is absorbed more ra])idlv on tlie side of its greatest abundance. 

 It undoubtedlv causes an osmotic swelling and tension, strong- 

 er on that side than on the other, and this difference is am]ile 

 to establish a line of chrertion towards which ro<^ts tiu'n in 

 their growth. Stems and leaves are obviously not hydrotropic 

 for they do not absorb any water. This fact illustrates the 

 adaptive character of responses; for it is a general ride that 

 plant parts are indifferent to stimuli to which there is no ad- 

 \antage in responding. 



It is said that when trees develop in a unifc^-mly moist 

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

 er drip of the fr)liage, which is obviously the place of greatest 

 dampness. But in case the soil is moister on one side than 

 .another, the roots grow more freel\- in that direction, and the 

 distance to which they will grow is sometimes almost incred- 

 ible. 



CHEMOTROPISM 

 Chemotropism is the sensitiveness of certain [ilant ''r- 

 gans by \ irtne of which tliey change their directions of growth 

 when acted upon by chemical substances. Roots will turn to- 

 wards a source of supply of some of the minerals they absorb. 

 The pollen tubes i.n their growth, tiu-n towards the substances 



