912 



ECOLOGY 



ticularly in stems and in roots, but it is in seeds that food accumulation 

 is most conspicuous, so that the chief discussion of plant foods has been 

 reserved for this place. Seeds are filled with food more generally than 

 is any other plant organ, and the kinds of foods reach here their greatest 

 diversity in composition, form, and distribution. The foods in seeds 

 and in other organs may conveniently be divided into those without 

 nitrogen (such as the carbohydrates) and those containing nitrogen 

 (notably the proteins). 



Starch. — Starch probably is more generally accumulated in seeds 

 and in other plant organs than is any other kind of food, being particu- 

 larly well known in the grains, in peas and beans, and in potato tubers. 

 Starch grains differ widely in size, in shape, and in structure, these dif- 

 ferences serving often to characterize particular species, genera, or 

 families (figs. 1206, 1211). As previously noted, starch grains are pro- 

 duced through the activity of plastids; in seeds the plastids concerned 

 are the colorless leucoplasts, the sugar that enters the developing seeds 

 being transformed by them into starch. As the starch accumulates in the 

 plastid, the peripheral portion of the latter expands until finally the pro- 

 toplasm consists merely of a thin film 

 enveloping the starch grain (see figs. 

 660-662).! 



The most obvious 

 structural features of 

 starch grains are their 

 lines of stratification, 

 which are due to alter- 

 nating layers of different 

 density (fig. 1207). In 

 the large grains of Pel- 

 lionia and probably else- 

 where the dense layers 

 have been thought to 

 represent accumulation 

 by day when the sugar 

 is abundant, the other layers representing accumulation by night 

 when the sugar supply is less (fig. 660). Thus the layers of starch 



Fig. 1206. — Cortical cells of a 

 potato tuber {Solatium tuberosum)^ 

 showing starch grains (s) of differ- 

 ent sizes, and also a protein crystal 

 (c) ; highly magnified. 



Fig. 1207. — A 

 starch grain from 

 the cortex of a 

 potato tuber (5o- 

 lanum tuberosum)^ 

 showing the eccen- 

 tric development of 

 t he ringsof growth; 

 very highly mag- 

 nified. 



* All gradations between ordinary chloroplasts and chloroplasts which are reduced to a 

 thin enveloping film often may be seen in the stems of various water plants, as Myrio- 

 phyllum. 



