32 BOTANY PART i 



grains contain only aleurone, and the inner almost exclusively starch, 

 it follows that flour is the richer or poorer in albumen, the more 

 or less completely this outer layer has been removed before the wheat 

 is ground. The aleurone layer remains attached to the inner layer 

 of the seed-coat, in the bran. 



Reactions for aleurone are the same as those already mentioned for the 

 albuminous substance of protoplasm. Treatment of a cross-section of a grain of 

 wheat (Fig. 24) with a solution of iodine would give the aleurone layer a yellow- 

 brown colour. 



ALBUMEN CRYSTALS. Crystals of albumen are of relatively frequent occurrence 

 in vegetable tissues and are often found in aleurone grains (Fig. 23) ; especially 

 large crystals are found in the endosperm of the Brazil nut (Bertholletia excelsa). 

 Albumen crystals may also occur directly in the cytoplasm ; as, for instance, in 

 the cells poor in starch, in the peripheral layers of potatoes, and in chromato- 

 phores (Fig. 28). Albumen crystals are sometimes found even in the cell nucleus. 

 This is particularly the case in the Toothwort (Lathraea), and in many Scrophu- 

 lariaceae and Oleaceae. Albumen crystals usually belong either to the regular 

 or to the hexagonal crystal system. They differ from other crystals in that, like 

 dead albuminous substances, they may be stained, and also in that they are 

 capable of swelling by imbibition. 



B. Inclusions of the Chromatophores 



Crystals of albumen and of pigments have already been mentioned 

 as occurring in chromatophores (Fig. 28), but the most important 

 inclusion is STARCH ( 29 ). The chloroplasts in plants exposed to the 

 light almost always contain starch grains (Fig. 15). These grains of 

 starch found in the chloroplasts are formed in large numbers, but as 

 they are continually dissolving, always remain small. Large starch 

 grains are found only in the reservoirs of reserve material, where 

 starch is formed from the deposited products of previous assimilation. 

 Such starch is termed RESERVE STARCH, in contrast to the ASSIMILA- 

 TION STARCH formed in the chloroplasts. It also only arises in 

 chromatophores, in this case the LEUCOPLASTS or starch-builders which 

 form it from sugar. 



All starch used for economic purposes is reserve starch. The 

 amount of starch contained in reservoirs of reserve material is often 

 considerable ; in the case of potatoes 20 per cent of their whole 

 weight is reserve starch, and in wheat the proportion of starch is as 

 high as 70 per cent. The starch flour of economic use is derived by 

 washing out the starch from such reservoirs of reserve starch. In 

 the preparation of ordinary flour, on the contrary, the tissues contain- 

 ing the starch are retained in the process of milling. 



The reserve starch consists of flat or roundish (oval or circular) 

 grains, differing in size in different plants. A comparison of the 

 accompanying figures (Figs. 25-27), all equally magnified, will give 



