DEVELOPMENT OF EMBRYO 363 



cotyledons, which are the first two leaves of the plant 

 (Fig. 62, D, E). Between them the terminal bud of the 

 primary shoot axis (epicotyl) is formed. At the other 

 end of the embryo, where it joins the suspensor, the 

 primary root is formed, the apical meristem just 

 inside the surface layer of cells which form the first 

 root cap. 



In the monocotyledons ^ the free end of the embryo 

 forms the single cotyledon, and the epicotyledonary 

 bud is developed laterally. The part of the primary 

 axis between the cotyledons and the primary root is 

 the hypocotyl. 



The embryos of different species vary very much in 

 the stage of development they have reached by the 

 time the seed is ripe. In some cases, particularly in 

 small seeds, the embryo remains in a very rudimentary 

 stage of development, surrounded by the endosperm. 

 In others it develops within the seed, not only large 

 leaf-like cotyledons; but an epicotyledonary bud with 

 the rudiments of several additional leaves. In such 

 large well-developed embryos the vascular system of 

 cotyledons, hypocotyl and primary root is all " blocked 

 out," so that the various tissues can be clearly recognised 

 in the embryo, though the cells remain small and without 

 any thickenings on their walls till germination. Very 

 often the seed grows to many times the size of the 

 ovule at fertilisation, as for instance in the bean. This 

 extensive development of the embryo requires of course 

 an ample supply of food, which is brought up through 

 the vascular bundles of the placenta and of the ovule 

 stalk to the developing seed. The embryo itself 

 grows at the expense of food absorbed from the endo- 



' Monocotyledons include the grasses, sedges, lilies, orchids, palms, 

 etc. 



