150 PLANT GROWTH 



tern, but all the other methods must be used to get the 

 maximum of value from the breeding work. Plant breeding 

 usually includes careful morphological studies of the whole 

 life history of the organism. 



Cytological studies are made when the facilities are avail- 

 able because this is a study of the chromosomes — the physi- 

 cal basis of heredity. Chromosomes are the smallest physical 

 structures that can be seen which are responsible for the type 

 of organism. Cytology is valuable in determining the num- 

 ber of chromosomes, and often in showing reasons for the 

 failure of crosses. Pollen grains, eggs, and even sperms may 

 be studied in this way. 



