10 



THE WHEAT PLANT 



From the point at the base of the grain where the latter was attached 

 to the rachilla, a delicate vascular bundle traverses the pericarp upwards 

 along the furrow to near the base of the style. 



The Seed. Within the pericarp is the seed, consisting of testa, nucellar 

 layer, endosperm, and embryo. 



(a) Testa. In a transverse section of a ripe grain of red wheat, the 

 testa or seed-coat, to which the colour of the grain is chiefly due, is seen 

 as a reddish-brown line immediately next to the " cross layer " (d, Figs. 

 3, 109), the " tube-cells " being usually invisible. It is derived from the 

 inner of the two ovular integuments, and consists of two layers of elongated 

 cells, the lumina of which have been obliterated by the crushing together 

 of the upper and lower walls ; the cell walls of the outer layer are almost 

 colourless, those of the inner one being tinted red or brown. On account 

 of their extreme tenuity the separate layers cannot be distinguished in 

 transverse sections of ripe grains, but after treatment of the pericarp with 



a solution of caustic potash, surface 

 views of them may be obtained, 

 when they appear composed of thin- 

 walled parenchymatous cells, 100- 

 150^ long and 15-20^ broad; the 

 long axes of the cells of the outer 

 layer cut the " cross layer " almost 

 at right angles, the inner cells being 

 arranged across the latter at an acute 



FIG. 6,-Surface.viewof the twoVell-layers an g le ( Fi S - 4 and 6 )- 

 of the testa ( x 210). i, The outer ; 2, the The seed-coat covers the embryo 



and endosperm, and is folded in 

 the centre of the furrow, where it joins the funicle of the seed. 



(b) The Nucellar Layer. Next to the reddish-brown seed-coat is a 

 bright colourless line the so-called " hyaline layer " possessing no 

 apparent cell structure (e, Figs. 3, 109). It is the epidermis of the nucellus, 

 the inner and outer walls of which have been crushed together, so that 

 the lumen of the cells is not visible in transverse sections. The cells are 

 more or less rectangular and oblong ; their outline may be seen on examin- 

 ing portions of the soaked pericarp. 



(c) The Endosperm. With the exception of the small space occupied 

 by the embryo the endosperm tissue fills the interior of the grain. It 

 may be divided into two parts, viz. : (i) the so-called " aleuron layer " 

 and (2) the starch- and gluten-parenchyma. 



The aleuron layer surrounds the rest of the endosperm, closely following 

 the contour of the seed-coat. It is almost entirely one cell thick, measuring 

 about 65-70 IJL across (g, Fig. 3) ; only in the furrow, and occasionally at 

 other parts, are two or more cells superposed. The individual cells are 



