PLANT REPRODUCTION 155 



paratively dry soil, the portulaca for example. Others 

 require abundant moisture ; among these plants are nelum- 

 biums and cannas. The larger number of seeds, however, 

 require a moderate amount of moisture, and it is important 

 that this amount- be constant, especially after germination 

 begins. A carnation-breeder may lose a season's work 

 simply by neglect of his seedlings at this critical period. 



Some seeds will germinate only under cool conditions, 

 for example the pansy ; others require a high temperature 

 in which to begin growth. Palms and other tropical 

 plants belong in this last class. 



Most species of plants have definite temperature re- 

 quirements for growth. A safe rule to follow is that the 

 seeds should be placed in the same temperature as that 

 required by mature plants of that particular species. 



The two factors, moisture and heat, are not sufficient 

 to produce germination. If seeds of some species are 

 planted deeply in warm, moist soil, they will decay. Ger- 

 mination never results. Protoplasm to be active must 

 have oxygen, and ordinary soil water rarely contains 

 enough free oxygen to allow germination to take place. 



118. Practical suggestions for seed sowing. In sowing 

 seeds, the following practical suggestions will increase the 

 number of seedlings from a given quantity of seeds : 



Compacting soil. Always compact the soil about seeds 

 to hasten germination. The seed coat will absorb water 

 much more rapidly if it is in direct contact with a moist 

 medium. If, however, the soil is very moist, as in seed 

 sowing out of doors in early spring, the soil should not be 

 compacted as much as later in the season when the surface 

 soil has become drier. 



Watering. Water seed boxes as soon as sown, pro- 

 vided they have not been watered by sub-irrigation methods 



