REPRODUCTION AND DISPERSAL 



915 



the gluten layer, is filled with small aleurone grains (fig. 1211); in most 

 other seeds the aleurone grains are scattered among the starch grains 

 or the drops of fat. Some- 

 times, as in Ricinus (fig. 1210), 

 the protein grains are large and 

 contain inclusions, such as pro- 

 tein crystals and globoids, the 

 latter composed in part of 

 calcium-magnesium phosphate. 

 Protein crystals also may lie 

 free in the cell sap, as in the 

 cortex of the potato tuber (fig. 

 1206) ; such crystals differ from 

 inorganic crystals in being able 

 to take stains and to swell in 

 certain media. In the algae, 

 nitrogenous foods occur in the 

 pyrenoids (fig. 106). 



The distribution of foods in 

 seeds and the associated advan- 

 tages. In nearly all seeds 

 there occur nitrogenous and 

 non-nitrogenous foods, the lat- 

 ter always dominating in amount. Commonly one form of non-nitrog- 

 enous food dominates in any given case, so that one may speak of 

 starchy, oily, or horny seeds. The percentage of carbon in fat is about 

 77 per cent, as compared with 44 per cent in starch, yet because of its 

 greater density, a given volume of starch contains about as much carbon 

 as does the same volume of fat. The chief advantage of fatty seeds 

 would seem to be that their relative lightness facilitates dispersal, while 

 on the other hand starchy seeds are better fitted for quick germination, 

 since the amount of oxygen required to make starch available for growth 

 is much less than that for fat. Thus it is distinctly advantageous that 

 large seeds generally are starchy ; where they are not starchy, germination 

 is very slow (as in the coconut). Among the seeds that are slow to 

 germinate are those with " reserve cellulose," as in the date. 



The influence of external factors upon the formation of accumulating foods. 

 Moderately high temperatures are favorable for seed maturation and also for maxi- 

 mum starch production, the optimum temperature for the latter being, in general, 



FlG. 121 1. A cross section through the outer 

 part of a wheat grain (Triticum sativum), show- 

 ing the husk (h) whose outer part is the peri- 

 carp and whose inner part is the testa, the aleu- 

 rone or gluten layer (a) whose cells are filled 

 with protein grains, and a part of the starch 

 region (6) which makes up the body of the grain ; 

 highly magnified. From COBB. 



