THE SEED, GERMINATION AND THE PLANTLET 31 



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 suffi- 

 ciently 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 neces- 

 sarily planted later, we must depend largely upon artificial 

 watering. 



51. Surface planting. 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 other- 

 wise, and must be watered often with a fine-rose watering 

 pot. When small seeds are sown in full exposure to sun- 

 light, 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 sphagnum moss that has been rubbed through a 

 sieve. This helps to retain moisture in the surface soil. 



52. Spores. Spores are the chief reproductive bodies 

 in plants that produce no seeds, as ferns, mushrooms, 

 mosses and the like. 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 de- 

 veloped plantlet (53) . Ferns are grown from spores sown 

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



