REPRODUCTION AND DISPERSAL 



913 



grains appear comparable to the growth rings of trees, like them being 

 caused by alternations in growth conditions.' Differences in size and 

 shape are due partly to growth conditions in the plastid. Commonly 

 growth starts in the center (as in peas and beans), and the rings are paral- 

 lel to the plastid periphery, the resulting grain being a symmetrical 

 spheroid or ellipsoid. Sometimes growth begins at one end, resulting 

 in eccentric rings (as in the potato tuber, fig. 1207). Sometimes more 

 than one grain forms in a plastid, resulting in a compound grain through 

 mutual crowding in growth (as in oats and rice) ; crowded grains often 

 are polyhedral in shape. 



The minute structure of starch grains is in doubt. One view is that they are 

 sphcrocryslals, that is, structures composed of a vast number of needle-like crystals 

 or trichiles, radiating in all directions from the growth center. This conception is 

 based upon tlieir behavior in polarized light, ^^'hich is comparable to that of inulin 

 when precipitated by alcohol (see below). Another view is that starch is an amor- 

 phous colloid; formerly this view was supposed to be supported by the fact that 

 starch grains readily absorb stains and swell as they absorb water, but certain un- 

 doubted crystals exhibit similar phe- 

 nomena. Starch grains, because of 

 strains arising from desiccation or 

 otherwise, often exhibit cracks radi- 

 ating from the center (fig. 1017). 

 The exact chemical formula of starch 

 is not known, but it is generally 

 written n (CsHioOs) (see p. 358). 



Variousnon-nitrogenous foods. 

 — Scarcely second in impor- 

 tance to starch among the non- 

 nitrogenous foods in seeds are 

 the fats or glycerids, which are 

 compounds of fatty acids and 

 glycerin, and are well illustrated 

 in the seeds of the castor bean, 

 cotton, and sunflower, and in 

 many nuts. The fats usually 

 exist as drops of oil in the cell 

 lumina. A third form in which non-nitrogenous food accumulates is 

 the so-called reserve cellulose or hemicellulose, which makes up the 



1 However, starch grains exhibit some stratification when exposed during development 

 to continuous illumination. 



Fig. 1208. — A section through part of the 

 endosperm of a persimmon seed (Diospyros 

 virginiana), showing greatly thickened walls 

 of "reserve cellulose"; the lines traversing 

 the cell walls indicate the paths of communi- 

 cation between adjacent cells; highly magnified. 



