to market at least two weeks earlier. Some plants can apparently be kept 

 working continuously, as they seem to need no "rest" or "sleep". 



Leaves as Starch Factories Common plants carry on photosynthesis 

 in a special organ, the leaf. The characteristic feature about leaves is the 

 blade, or flat and comparatively thin structure. Some leaves have stalks, or 

 petioles; and all have "veins" running through the blade. Leaves vary 

 remarkably in size, shape and the character of the edges and of the surface 

 (see illustration, p. 43). Some are hairy; others are quite bald. Even the 

 color is not uniform, for the chlorophyl varies in density, and the appear- 

 ance is influenced by other pigments, air spaces, wrinkles, hairs, and other 

 details. And many "leaves" depart widely from our ordinary notion of 

 what a leaf is. Some are hardly more than stiff bristles, as on certain cac- 

 tuses. Others have sensitive extensions, or tendrils. In some species the 

 leaves are more or less active in capturing animal food (see page 542). But 

 starch-making proceeds in about the same way in all leaves containing 

 chlorophyl (see illustration, p. 139). 



Transpiration^ Water evaporating from the leaves sets up currents that 

 distribute throughout the plant water and salts absorbed from the soil. 

 This loss of water, or transpiration, is at the same time a source of danger 

 to the plant, for more plants die from wilting than from any other one 

 cause. 



iSee Nos. 5 and 6, p. 158. 



I. p. Flory. Boyce Thompson Institute 



LIGHT AND CHLOROPHYL 



Normal seedlings grown in the light appear green from the start. Seedlings kept in 

 the dark remain white until after they are placed in light. Albino plants never de- 

 velop chlorophyl, and wither when the seed nutriment is exhausted 



140 



