20 SIMPLIFIED LEAVES 



complicated in form. The root-system consists of a short 

 primary root (/-), with a peculiar sheath, and several small 

 lateral roots. The axis (ft) above the primary root bears 

 a number of alternate sheathing-leaves. But one character- 

 istic feature of the embryo of this and other grasses is 

 that, attached to the hypocotyl, is a shield-like outgrowth — 

 the scutellum (sc) — which separates the rest of the embryo 

 from the endosperm. Botanists are not agreed as to which 

 portion of the embryo represents the cotyledon. There 

 are the three following views : — (i) The scutellum is the 

 cotyledon. (2) The first sheathing-leaf and the scutellum 

 together constitute the cotyledon. (3) The scutellum is 

 merely a " subsidiary outgrowth " (emergence) of the hypo- 

 cotyl : and the first sheathing-leaf represents the cotyledon. 



Comparison between a Wheat-grain and a Bean-seed and 

 their Germination. 



Grain of Wheat. 



1. Is a fruit which contains one 

 seed. The wall is composed of the 

 fruit- wall (pericarp), together with a 

 very thin testa, which can be distin- 

 guished only with the help of a 

 compound microscope^* 



2. The space within the testa is 

 occupied by endosperm and an 

 embryo. The seed is consequently 

 described as endospermic. 



3. Embryo has one cotyledon. 



4. In germination. 



(a) The endosperm shrivels as 



the seedling develops ; it is 

 the food - supply of the 

 embryo. The scutellum 

 is the sucking-organ which 

 absorbs nutriment for the 

 benefit of the young plant. 



[b) The main root forces its way 



through a, peculiar root- 

 sheath, 

 (f) The main root soon ceases 

 to grow, and adventitious 

 roots arise on the stem. 



Bban-Seed. 



I. Is a seed. 



Its wall is formed by the thick 

 testa only. 



J.. The space within the testa is 

 occupied by an embryo only. There 

 is no endosperm. The seed is said 

 to be non-etidospermic. 



3. Embryo has two cotyledons. 



4. In germination. 



(a) The cotyledons shrivel as 

 the seedling develops. 

 They contain the food- 

 supply of the embryo. 



(A) The root has no peculiar 

 root-sheath. 



(f) The main root continues to 

 develop and produces 

 acropetal lateral roots. 



* Recent investigations tend to show that, in reality, the testa is entirely 

 destroyed before the grain is ripe. 



