96 SEEDING AND PLANTING 



lings. When seedlings from large seed are grown in competition 

 with seedlings from small seed, they exhibit greater resistance to 

 external injurious influences, are of more vigorous growth, and are 

 able to crowd out less vigorous seedlings from small seed. The 

 impetus given to seedlings from large seed is not temporary, as 

 they usually form the dominant trees in the stand. 



There is considerable variation in all species in the weight of a 

 given volume of seed. This is particularly true of commercial 

 seed, largely due to the variable amount of foreign matter that 

 it contains. The weight, however, is not uniform even when the 

 seed is equally clean and free from impurities. The variation in 

 the weight of clean seed is chiefly due to gathering the seed before 

 maturity and differences in seed-filling, i.e., to shriveled or unde- 

 veloped kernels arising from adverse conditions for development. 



Cjl VARIATIONS IN SEED FILLING 



By the examination of 100 seeds of a given species we may find 

 70 of them with plump, well-developed kernels and the remaining 

 30 with shriveled or otherwise undeveloped kernels. In the latter 

 for one reason or another the embryo failed to develop. The per- 

 centage of filled seed under normal conditions depends upon the 

 species, the season and the vigor of the mother trees. Old and 

 overmature trees produce very poorly filled seed. The best filled 

 seed is obtained from middle-aged, normal trees. 



Pollenation, which in most species takes place in late spring or 

 early summer, is favored by a dry spring and by early summer 

 winds. Under these conditions the pollen is carried more abun- 

 dantly by both insects and wind, and there is a better filling of 

 the seed. A warm, moderately dry spring also increases the de- 

 velopment and deposition of reserve food material in the seed and 

 hastens its ripening. There are, therefore, fewer unripe seeds in 

 the autumn. Unripeness is a large factor in causing the seed to 

 be overlight in weight. 



8. THE AVERAGE NUMBER OF SEEDS PER POUND 



Not only is there great variation between different species in the 

 size and weight of the seed, but there is also more or less variation 

 within the species. 



The differences in the size of seed within the species is due to 

 individual variation, occasioned by differences in the vigor and age 



