THE GRAIN 



ii 



rectangular in transverse and longitudinal sections, and polygonal, with 

 rounded corners in surface view, varying in diameter from 25 to 75 /t, 

 and having walls about 6 /x thick (Fig. 7). In front of the scutellum the 

 layer consists of smaller cells, while over the plumule and coleorhiza it 

 becomes so very thin and shrivelled that it is difficult to recognise it as 

 a separate layer. Formerly the cells were termed " gluten cells," but 

 Schenk in 1872 showed that they contain no gluten. In each is a round 

 or oval nucleus about 12 \L in diameter and possessing three or four nucleoli. 

 The rest of the cell cavity is filled with minute spherical bodies, usually 

 described as aleuron grains, im- 

 bedded in a small amount of waxy 

 or oily cytoplasm. Each grain 

 measures about 3 or 4 /z in dia- 



*,- 



FIG. 7. Surface view of the aleuron 

 layer ( x 210). 



FlG. 8. Cell of the aleurtm layer after 

 treatment with dilute alcohol, showing 

 nucleus and insoluble membranes of the 

 " aleuron grains " ( x 770). 



meter, and consists of a highly refractive core surrounded by a thin 

 membrane. When treated with water, salt solution, or weak acids or 

 alkalies, the core dissolves, leaving the insoluble membrane as a hollow 

 ball (Fig. 8). O'Brien considers that the homogeneous core consists of a 

 soluble protein which, according to Groom, contains a small amount of 

 magnesium and calcium phosphates, but neither globoid nor crystalloid as 

 found in typical aleuron grains are present in these granules. 



The starch- and gluten-parenchyma, forming by far the greater portion 

 of the wheat grain, consists of thin-walled polyhedral cells, with their 

 long axes arranged usually at right angles to the surrounding pericarp. 

 Adjoining the aleuron layer, they are comparatively small, becoming two 

 or three times as large in the central parts of the endosperm. In contact 

 with the back of the scutellum of the embryo the cells of the endosperm 



