134 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 
Fig. 38. Ecads of Ad/ionia linearis, showing position i 
of chloroplasts. The palisade shows spostieehe, the hence with Response: £0 
spans See 1, eae (chresard, 2-5, light, 1); light stimuli. Aeration 
2, shade leaf (chresard, ; light, .012); 3, shade leaf i i 
(chresard, 114; light, .003). > 250. 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 
