528 ECOLOGY 



rather high (from 20° C. to 22° C. in many plants). The rate of carbo- 

 hydrate synthesis varies also with the carbon dioxid available, the 

 amount commonly present being too low for optimum synthesis. There 

 is ground for believing that an optimum amount of carbon dioxid has 

 been present in certain geological ages, which might account for the ex- 

 treme luxuriance of the vegetation then existing. In dry soil the water 

 supply may be insufficient for optimum synthesis. It is probable that 

 temperature, carbon dioxid, and water, more often than light, are limiting 

 factors. In the Mediterranean region, winter synthesis is slight because 

 of low temperature, while in summer it is slight because of desiccation; 

 since the stomata close in dry weather, desiccation is likely to result 

 in a decrease of carbon dioxid as well as in a decrease of water. 



Starch formation. — The synthetic process above outlined culminates 

 in the formation of sugars, which readily pass as solutes from cell to cell. 

 During the day sugar is manufactured more rapidly than it can be trans- 

 ported, and the excess, for the most part, is converted into starch, which 

 accumulates in the chloroplasts. During the night this starch is recon- 

 verted into sugar and removed from the working cells. Thus the 

 accumulation of starch in the chloroplasts is a measure, not, as has been 

 thought, of the working capacity of a leaf, but of the excess of sugar- 

 making over sugar transportation. Starch-making is a function of the 

 plastids, not only of the chloroplasts but also of colorless plastids (leuco- 

 plasts); it takes place in the dark {e.g. in potato tubers) as well as in 

 the light. Starch manufacture is favored by high rather than by low 

 temperatures; hence, as might be expected, sugar generally replaces 

 starch in winter leaves. 



The synthesis of proteins. — Comparatively little is known concerning protein 

 synthesis, which seems to be a process common to all plants. The chief necessity 

 is a supply of carbohydrates and of available nitrogen. In green plants the chlo- 

 rophyll-bearing cells probably are the chief seat of protein synthesis as of carbo- 

 hydrate synthesis, and the process seems to take place chiefly in the light, though 

 neither light nor chlorophyll is necessary if the carbohydrate supply is adequate. 



Anthocyan. — General features. — Contrasting with chlorophyll, xanthophyll, 

 and carotin, which occur in plastids, are the red pigments (anihocyans) of many 

 leaves, which usually are dissolved in the cell sap (occasionally occurring as crystals 

 or grains), and thus are disseminated uniformly throughout the cells where they 

 occur. They are most famiUar, perhaps, in dying autumn leaves, but are common 

 in young leaves (especially in spring), and occur at times in winter leaves and in 

 shade leaves (especially beneath), while in some plants (as Coleus) they are always 

 present. The pigments may be in the epidermis (as usually in floral leaves and in 

 developing foliage leaves), or they may occur in the chlorophyll-bearing cells (as 



