'34 



THE PLANT 



regular response to the action of light that it is regarded as the normal in- 

 dication of photosynthetic activity. The mere presence of chlorophyll is 

 not an indication of the latter, since chlorophyll sometimes persists in light 

 too diffuse for photosynthesis. The amount of starch formed is directly 

 connected with the light intensity, and in consequence it affords a basis for 

 the quantitative estimation of the response to light. Two responses to 



light stimuli have a 

 direct effect upon the 

 amount of transpiration. 

 Of the light energy 

 absorbed by the chloro- 

 plast, only 2.5 per cent 

 is used in photosyn- 

 thesis, while 95-98 per 

 cent is converted into 

 heat, and brings about 

 marked increase in trans- 

 piration. Furthermore, 

 in normal turgid plants, 

 the direct action of light, 

 as is well known, opens 

 the stomata in the morn- 

 ing and closes them at 

 night. 



175. Aeration and 

 translocation. The 



movements of gases and 

 of solutions through the 

 tissues of the leaf are 

 intimately connected 

 with photosynthesis, and 

 hence with responses to 

 light stimuli. Aeration 

 depends primarily upon 

 the periodic opening of 

 the stomata, for, while the carbon dioxide and oxygen of the air are able to 

 pass through epidermal walls not highly cutinized, the amount obtainable 

 in this manner is altogether inadequate, if not negligible. The development 

 of sponge tissue or aerenchym is intimately connected with the stomata. 

 The position and amount of aerenchym and the relative extent of sponge 

 cells and air-spaces are in part determined by the number and position of the 



Fig. 38. Ecads of Allionia linearis, showing position 

 of chloroplasts. The palisade shows apostrophe, the 

 sponge epistrophe: 1, sun leaf (chresard, 2-5^, light, 1); 

 2, shade leaf (chresard, \\%\ light, .012); 3, shade leaf 

 (chresard, \\%\ light, .003). X 250. 



