20 THE NURSERY-MANUAL 



the double purpose of preventing too great heat and too rapid 

 evaporation. Various covered seed-boxes are used for the 

 purpose of maintaining approximately the required tempera- 

 ture, but as they are oftener employed in bud-propagation, 

 they are discussed in that connection (Chapter V) . 



Bottom heat is helpful to germination in most seeds, but, 

 except in the case of certain tropical species, it should not be 

 strong. It is a common practice to place seed-boxes on moder- 

 ately cool pipes under benches in a greenhouse. Seeds of 

 hardy annuals and perennials do not require bottom heat, 

 although they may be benefited by it. If the earth in seed- 

 beds should become too cool, watering with warm or tepid 

 water may be helpful. 



It is impossible to give rules for the proper temperature for 

 different kinds of seeds. In general, it may be said that seeds 

 germinate most rapidly at a temperature a few degrees above 

 that required for the best development of the plant itself. 

 Seeds of hardy plants require a temperature of 50 to 70, 

 conservatory plants 60 to 80 and tropical or stove plants 

 75 to 95. The plantlets should be removed from these 

 highest temperatures, as a rule, as soon as germination is 

 completed. 



In outdoor culture, depth of planting has a direct relation 

 to temperature. Seeds may be planted deeper late in the 

 season than early when the soil is cold and damp. Deep 

 planting probably as often kills seeds because of the absence 

 of sufficient warmth as from the lack of oxygen or the great 

 depth of earth through which the plantlet is unable to push. 



Influence of light on germination 



The influence that light exerts on germination is not definitely 

 understood for all horticultural seeds. It is known, however, 

 that seeds will often germinate in full sunlight, if the proper 



