The Plantlet. 39 



them in contact with moisture (29). Since the plantlet 

 must force its way through the soil that covers the seed, 

 the less the depth of this soil, other things equal, the less 

 energy and the shorter time are required for the plantlet 

 to reach the surface. Therefore, seeds should not be planted 

 deeper than is necessary to insure the proper supply of moisture. 

 Small seeds, as of lettuce, celery and carrot, produce 

 such weak plantlets that it is unsafe to cover them suf- 

 ficiently to insure the proper moisture supply in dry 

 weather. We must, therefore, plant such seeds so early 

 in spring that the soil has not had time to become dry, 

 or if necessarily planted later, we must depend largely 

 upon artificial watering. 



52. Very Small Seeds, as of petunia and tobacco, Should 

 Not Be Covered with soil at all, but may be pressed down 

 into fine loam with a board or otherwise, and must be 

 watered often with a fine- rose watering-pot. When small 

 seeds are sown in full exposure to sunlight, it is well to 

 shade the surface with paper or a muslin-covered frame, 

 to check evaporation until the plantlets appear. Small 

 seeds are sometimes covered with a thin layer of sphag- 

 num moss that has been rubbed through a sieve. This 

 helps to retain moisture in the surface soil. 



53. Ferns are Grown from Spores* sown on the surface 

 of fine soil in a propagating frame (369), in which the 

 air is kept very moist and the surface of the soil never 

 becomes dry. 



* Spores are the chief reproductive bodies in plants that produce no 

 seed, as ferns, mushrooms, mosses, etc. They are usually so small as to 

 be barely visible to the unaided eye. The dust that escapes from a puff- 

 ball when it is squeezed or from a bunch of corn smut is formed of the 

 spores of these plants. Spores usually consist of a single cell, in which 

 respect they differ materially from seeds, which contain a more or less 

 developed plantlet (54). 



