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A TEXTBOOK OF BOTANY [Cn. 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. v 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- 



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 ^. (Modified from Errera 

 and Laurent.) 



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



