490 GRAMINE^. CEREALS 



self-pollination results. In wheat, barley and oats self-pollina- 

 tion is followed by fertilisation and the production of fertile 

 seeds, whereas in rye self-pollinated flowers are almost always 

 sterile and considerable decrease of yield results to the crop 

 when damp, dull weather prevails for a long time and prevents 

 the proper opening of the flowers and distribution of the pollen. 



In wheat the first flowers to open are those situated about one- 

 third of the way from the apex of the ear, the rest follow in 

 succession upwards and downwards from this point. Each 

 flower remains open from a half to one hour, and the whole 

 ear completes its flowering usually in eight or nine days. 



In barley the complete period of flowering of the ear is shorter, 

 and when the flowers open they remain expanded a longer time 

 than those of wheat. Frequently, however, the flowers of barley 

 never open at all and self-fertilisation is the rule. In fact all the 

 cereals, except rye, are generally self-fertilised, although natural 

 crosses among wheats and among oats have been observed 

 occasionally. Hybrids between rye and wheat have been pro- 

 duced. 



Ripening. — A few hours after pollination the pollen-tube 

 reaches the ovule, and fertilisation of the ovum is effected. The 

 latter then gradually becomes an embryo and in the embryo-sac 

 around it the formation of endosperm-tissue takes place. 



Within the endosperm-tissue there is also a gradual accumula- 

 tion and storage of proteids and carbohydrates. 



Some of the proteids are laid down in the outermost layer of 

 cells of the endosperm in the form of aleuron-grains, and it is 

 important to note that the filling of the aleuron-layer of cells 

 takes place before the central portions of the endosperm are 

 completed. In the small and rapidly-ripened grains of cereals 

 the percentage of proteids in comparison with the carbohydrate 

 starch is therefore higher than in slowly-matured plump grains in 

 which time has allowed of the extended accumulation of starch 

 after the aleuron-layer has been laid down. 



