14 THE WHEAT PLANT 



(iii.) The coleoptile or plumule-sheath is a protective leaf, which com- 

 pletely encloses the plumule, except near the apex, where, on the side 

 opposite to the scutellum, there is a minute opening (Figs. 9 and 10), 

 through which the first green leaves of the plumule ultimately make their 

 way. It springs from the same side of the axis as the scutellum and 

 immediately above the point of insertion of the latter organ. In the 

 resting embryo it is short about i to 1-25 mm. long but as it pushes 

 its way upwards through the soil when the grain is sown, it lengthens, 

 and may grow to a length of i or 2 inches. 



(iv.) The plumule within the coleoptile consists of two or three rudi- 

 mentary leaves surrounding the growing point. These have a divergence 

 of ^, the first being on the side of the axis opposite the scutellum. In 

 addition to the primary bud there is in the wheat embryo a secondary 

 lateral bud in the axil of the coleoptile on the side next to the scutellum 

 (d, Fig. 10). 



(v.) The Root-system. The embryo in a grain of Bread wheat possesses 

 a well-differentiated root-system, consisting of five rootlets, namely, a 

 primary radicle directed slightly forwards and two pairs of secondary 

 lateral rootlets, which arise from the short axis of the embryo in a plane 

 almost parallel to the face of the scutellum. 



The roots of the lower older pair, which arise just above the point of 

 insertion of the epiblast, are not so large as the primary radicle ; those 

 forming the upper younger pair are very much smaller. Another single 

 rootlet often grows out in front above the epiblast between the lateral 

 pairs, but it is rarely differentiated in the resting embryo of wheat, although 

 it frequently appears after germination is well advanced (b, Figs. 18, 22). 



Later still, a pair of opposite rootlets arise above the other pairs and 

 parallel to them, the seedling plant then possessing eight rootlets. 



Each of the rootlets terminates in a growing point covered by a root- 

 cap, and the primary and two lowest pairs just mentioned are enclosed 

 within the coleorhiza or root-sheath, a parenchymatous tissue connected 

 with the hypocotyl and the lower half of the scutellum. 



(vi.) The Vascular System. The vascular system is clearly defined 

 in the resting embryo in the form of procambial strands, consisting of elon- 

 gated thin-walled conducting cells of polygonal section, each from 50 to 

 80 /A long and 6 to 8 ju. in diameter. 



A broad strand, oval or elliptical, in transverse section (Figs. 13 and 

 14), curves out almost at right angles from the hypocotyl, and extends 

 upwards in the free half of the scutellum in a median plane, nearly to the 

 upper edge (Fig. 10). For a short distance it becomes broader in its 

 progress upwards, but finally divides and spreads out in a fan-like fashion, 

 giving off a number of fine branch strands, each five to ten cells thick. 

 Some seven or eight of these on the right and a similar number on the 



