152 



A. H. Graves, 



tuberance is really the growing point or the third leaf, but a com- 

 parison with the growing point of the mature stem (PL I, fig. 1 ;- 

 Text-fig. 2, p. 75) conclusively proves its nature. 



The Hypocotyl. The great bulk of the embryo is taken up by the 

 hypocotyl (PI. XV, fig. 119, Text-fig. 28), which is nothing but a 

 mass of storage tissue, its cells being gorged with large starch grains. 

 Even in such a young stage as in PI. XIII, fig. 105, the cells in 

 this region are distinctly larger than in the remainder of the embryo. 

 They continue their enlargement and acquire an ever richer content 

 of starch as development proceeds (cf. PL XIII, figs. 105, 106 ; PL XIV^ 

 figs. 110-112, 114; PL XV, fig. 115). 



-hyp 



Figure 28.— Mature embryo dissected from ripe seed, tlie cotyledon slightly 



elevated during manipulation, cot^ cotyledon ; epc^ region of epicotyl ; adr^ 



adventitious root ; hyp^ hypocotyl. x 35. 



The Primary Root. At a fairly early period (PL XIII, fig. 106 ; PL XIV, 

 fig. 110) a certain group of cells becomes difi'erentiated at the base of 

 the embryo, by acquiring denser contents and lacking the starch grains 

 of the storage cells above them. Each of these cells later develops 

 (at least those on the periphery of the embryo) into unicellular 

 papilla-like projections, essentially as figured and described by 

 Wille (1883, p. 4 and PL I, fig. 19) and Murbeck (1902, pp. 17, 18 

 and figs.^ 60 b, 61 b). Like Murbeck, I find also often more than 

 a single layer, but whether arising from periclinal divisions in the 



