THE GRAIN OF WHEAT. 419 



mis of the pericarp. This is all that remains of the pericarp. The 

 tissue next following appertains to the seed. It is separated from 

 the pericarp by more or less numerous air spaces. The spermo- 

 derm shows first a thin, apparently homogeneous, colourless skin, 

 arising from a layer of cells with obliterated cavities ; to this 

 follows a similar narrow layer, the scarcely recognisable cavities of 

 which have brown contents. These two layers are represented in 

 ii, in Fig. 151, A ; together they constitute the spermoderm ; the 

 outlines of their cells are recognisable upon tangential sections. 

 All the elements of pericarp and spermoderm are, so far as they 

 possess cavities, filled with air. To the spermoderm succeeds a 

 relatively thick, strongly refractive, white skin (n), which- owes 

 its origin to the outermost layer of the nucellus. The original 

 cavities of the cells in this are indicated by harrow', granular, 

 tangential striae. To this layer follows the layer of radially- 

 elongated aleurone-containing endosperm cells (al), which we 

 have already studied in Chap. II. ; and next to these come the 

 cells of the inner endosperm, filled with starch. The wall of the 

 furrow shows the mesocarp much increased in quantity, and the 

 layers progressively of larger cells ; while in the middle line the 

 elements are again small, thin-walled, without interstices, and 

 have a median, feebly-developed vascular bundle. The layer chl 

 is thicker, and contains chlorophyll and large air-cavities ; the 

 outer nucellar . layer thickens into a cushion of tissue, behind 

 which the aleurone layer is often wanting. This increased de- 

 velopment of the outer tissues is accompanied by a deeper grooving 

 of the endosperm than of the spermoderm and pericarp, and a 

 sharp limit between the tissue of the seed and of the fruit is 

 not recognisable in the furrow. 



Let us now endeavour to obtain a tolerably median longi- 

 tudinal section through the ripe grain, for which purpose we 

 must not, however, use air-dry, but softened, or even better, 

 grains which have only just ripened. The embryo will show 

 specially well in sections examined in carbolic acid, or treated 

 with potash and examined in glycerine. We will first examine 

 the section with a low power, and only use a high power for the 

 detailed study of individual parts. We begin with the embryo. 

 This is situated obliquely, at the base of the body of the endo- 

 sperm (Fig. 151, B), and is in contact with it by means of the 

 shield-like scutellum (sc). The scutellum appears in longitudinal 

 section as a flat structure, which at both upper and under termi- 



