FORM AND STRUCTURE 9 



least possible, and both probably occur. In all barleys, 

 however, cross-fertilization is rare, and in many two-row 

 varieties apparently impossible. On the other hand, cross- 

 fertilization is the rule in rye. Wheat and oat varieties 

 are usually self-fertilized, but there are adaptations for 

 cross-fertilization, and no doubt the latter occurs much 

 more frequently than is generally supposed. It is certain 

 that there are numerous natural crosses among durum 

 wheats and emmers. Also common wheats no doubt 

 cross with these groups naturally, thus reducing these 

 striking intermediate forms sometimes seen in the field 

 which are otherwise unexplained. 



8. The ovule. Without a funiculus, the ovule grows 

 to the ventral suture of the carpel, and is hemitropous, 

 having the micropyle facing downward and outward. 

 There is an outer and inner integument, and along the 

 anterior side of the former, the pollen tube passes to the 

 micropyle. The embryo sac makes rapid growth at once 

 after fertilization, and in the meantime there is stored up 

 within it an abundant supply of food material, especially 

 a large amount of starch. The ovule increases in size 

 until it completely fills the cavity of the ovary and finally 

 merges with its walls. The embryo, at first club-shaped, 

 develops its upper portion into a cotyledon, which becomes 

 shield-shaped. From the epicotyl arises externally a 

 collar-like projection upward, the beginning of the coleop- 

 tile. The foundation of the radicle at first lies deep in 

 the lower portion of the embryo and grows with it, but 

 later becomes separated from the surrounding tissue, 

 which latter forms the coleorhiza or root-sheath, corre- 

 sponding to the coleoptile. Other radicles soon arise at the 

 base of the main radicle, while the beginnings of foliage 

 leaves in the upper embryo increase to 3 or 4 (Fig. 6). 



