GKEAT VAKIABILITY OF TOBACCO PLANTS. 11 



seed plants have been cut off while many of the flowers were still in 

 bloom. On such seed heads seed pods in all stages of maturity can 

 be found. Some of the pods are fully ripe and contain mature seed, 

 while others have not fully developed. Much of the seed is imma- 

 ture and contains little food for the nourishment of the plantlet. 

 These seed heads are frequently thrashed out with a flail or the pods 

 are crushed by the hands in order to shell out the seed. In this way 

 the immature seed is mixt with the ripe seed sown in the seed 

 beds. In the seed beds the immature seed frequently sprouts earlier 

 than the mature seed, and the early seedlings grown from such seed 

 ai*e naturally used for transplanting in the field. Such plants have 

 a great tendency to vary, in some cases being very early, and as a 

 rule having leaves that are small, coarse, and wholly undesirable for 

 any purpose. These weak, immature tobacco seeds, according to 

 careful and extensive observations by the writers, produce plants 

 which are more subject to certain diseases, particularly the mosaic 

 disease, than are plants grown from mature seed. 



The excess of plant food in the soil where heavy applications of 

 barnyard manure and commercial fertilizers are used is usually 

 thought to produce variations in the plants. This variation is usually 

 shown by an increase in the size of the leaves, which is generally 

 correlated with changes in color, flavor, and other characters. In 

 these cases there is usually a tendency for the type of plant to break 

 up, so that the uniformity of the crop is disturbed. Where it is 

 necessary to use large quantities of fertilizers in the growing of a 

 profitable crop, the inclination to variation induced by this intensive 

 system of cultivation must be controlled by the most rigid selection 

 of seed from the type of plants best adapted for the purpose for 

 which the tobacco is grown. 



The change of soil and climatic conditions, particularly the taking 

 of tobacco seed from southern or tropical conditions to the north, 

 is a fertile source of variations in tobacco. The fixation of a uniform 

 type in this case requires several years of acclimatization, supple- 

 mented by selection of seed from the desirable plants. 



In the production of improved varieties of tobacco by breeding, va- 

 riation in type can be secured by crossing, and by continued saving of 

 self-fertilized seeds from plants most nearly reaching the growers' 

 ideal of perfect plants uniform types can be fixt. Growers will fre- 

 quently find plants that are markedly better than the rest of the plants 

 in the field, so that by selecting these desirable variations a steady im- 

 provement in the yield and quality of the crop can be effected. Vari- 

 ation, therefore, is a basis for selection in an experimental way, but 

 in practise every effort must be put forth in order to secure uni- 



96 



