Ways in Which Plants Increase in Size 



335 



field to itself. A part depends on direct injury done by bright 

 light through the injurious chemical reactions set up in the com- 

 plicated protoplasm, a matter we have considered pretty fully 

 under Protection. On green plants, of course, the action of light 

 is far less injurious than on colorless kinds, because the chloro- 

 phyll incidentally forms an excellent protective screen. In chief 

 part, however, the lesser growth of plants in light is due to the 



FIG. 125. Pots of Scilla, started alike; but that on the right was kept in a dark room. 



great promotion of transpiration by the light and its associated 

 heat, whereby so much water is removed from the plant as to lessen 

 the supply available for swelling the growing cells, for such swell- 

 ing is essential to their growth, as will be noted more fully a few 

 pages later. 



Thus, it is plain that light, like heat, can become too strong for 

 the best growth of plants. We have seen already that even in 

 photosynthesis plants cannot make use of all the light supplied by 

 direct bright sunlight. These facts together explain why so many 



