160 



A TEXTBOOK OF BOTANY 



[Ch. IV, 7 



self-adjustment, tending to raise the green tissues into better 

 light conditions, when the plants happen to start in dark 



places. 



4. Many minor condi- 

 tions likewise affect growth, 

 notably special gases in the 

 atmosphere, peculiar condi- 

 tions of soil and water, 

 electrical currents, or baro- 

 metrical pressure. Some of 

 these conditions accelerate 

 growth, while others retard 

 it, though mostly in minor 

 degree, and often in devi- 

 ously indirect ways. 



All growth requires, in 

 addition to warmth and hu- 

 midity, a supply of water, 

 food, and air. The water, 

 absorbed by the roots, is 

 needed for the swelling of 

 the young parts. The food, 

 whether made in the green 

 leaves or absorbed from 

 some other source, is needed 

 partly as building material 

 for the enlargement of cell 

 walls, and partly as a 

 store of energy for effecting 

 the plant work. The air, 

 more properly the oxygen, 

 is needed to release the 

 energy in the food through 

 the action of respiration, a 

 fundamentally indispensa- 

 ble process, considered in the following section. As by- 



Fig. 112. — The place of elonga- 

 tion in growth of a typical stem of 

 Lysimachia vulgaris, as shown by 

 the spread of marks at first evenly 

 spaced ; X 2- (Modified from Errera 

 and Laurent.) 



