'34 



THE WHEAT PLANT 



and in a few days a central core of cells with a surrounding dermatogen 



layer is recognised (4, Fig. 106). 



Soon the embryo becomes a club-shaped mass of cells with a narrow 



elongated base, and after reaching a length of -3- -4 mm. the first sign of 



differentiation is visible ; a lateral notch or depression within which the 



growing -point is situated (, 

 Fig. 1 06) appears on the side 

 of the embryo opposite to the 

 endosperm. 



From the distal portion of 

 the embryo above the notch 

 the scutellum arises ; this is a 

 terminal structure, which I con- 

 sider the cotyledon. 



By the time the embryo 

 has reached a length of about 

 5 mm. the coleoptile appears 

 in the form of a ring surround- 

 ing the stem apex, the upper 

 half of the ring developing 

 more rapidly than the lower 

 part, and appearing in longi- 

 tudinal section as a curved 

 scale covering the growing- 

 point. Later, when the embryo 

 is about i mm. long, the rudi- 

 mentary first foliage leaf arises, 

 the plumule consisting at this 

 6 stage of (i) the short and some- 



FIG. 1 06. Longitudinal sections of young em- what cone - shaped coleoptile 

 bryos. a, Growing-point ; 1-5 ( x 210) ; 6, w ith a circular terminal open- 

 ing, (2) the rudimentary foliage 

 leaf alternating with the coleoptile, and (3) the stem apex within. 



Growth of the epiblast now begins. 



After this stage rapid intercalary growth commences at the base of the 

 scutellum near the point of origin of the coleoptile. The upper part of 

 the scutellum which I regard as the blade of the cotyledon is lifted away 

 from the plumule, thus allowing for the subsequent unhindered growth 

 of the latter. 



The rapidly lengthening portion represents the sheath of the scutellum, 

 the whole development being similar to that of the foliage leaves of grasses 

 generally. 



At first the sheath of the scutellum is thin ; later it becomes thicker, 



