Leaves. 



91 



from the sun, the chloroplasts cease to manufacture starch, 

 and, gradually becoming emptied of what they contain, by 

 sunrise may show no further traces of it. Thus they begin 

 each day's work unhampered by the products of their 

 previous labors (see Fig. 39). 



68. Manufacture of Proteids. — Starch and sugar repre- 

 sent only one class of food materials necessary to the 

 nutrition of plants; the 

 proteids, which contain 

 .litrogen and sulphur and 

 sometimes phosphorus 

 in addition to the carbon, 

 hydrogen, and oxygen 

 composing starch and 

 sugar, have yet to be 

 accounted for. Proteids 

 can be formed in any of ^ . , , , , 



■> _ Cross section through leaf of MelUotus alba 

 the living cells, and in takenjustbeforesunrise, treated with chloral 



darkness as well as in "^^^T^ '""^'"^ and showing that the starch 



has been removed during the night. 



light. They are evi- 

 dently formed by the cytoplasm of the cells from the 

 elements of starch, combined with the compounds of nitro- 

 gen, sulphur, and often phosphorus, which have been 

 abstracted from the soil by the roots. Since their manu- 

 facture can proceed in darkness, the energy for their 

 production is evidently derived from the oxidation of 

 sugar, and other soluble compounds obtained from the 

 starch. 



69. Transpiration, and Evolution of Oxygen. — Only a 

 small per cent of the water entering the palisade cells is 

 used in the manufacture of starch, the greater bulk of it 

 being transpired into the intercellular spaces, and passing 

 out of the leaf through the stomata. The openings in the 



