36 HEAT. 



and thus shades the plant from the otherwise de- 

 structive rays of the sun. 



5. Heat is an important stimulus to vegetable 

 life. The functions of most plants cannot be per- 

 formed without a certain degree of this agent. The 

 more rapidly the tree is growing, the more is it de- 

 pendent on the maintenance of a high temperature. 

 The requisite degree, however, differs widely with 

 the species, or even variety. Mr. Lymburn speaks 

 of a plant of Marchantia which was growing in a hot 

 spring on the island of Amsterdam, where the water 

 was above the boiling-point; while, on the other 

 hand, the curious Protococciis nivalis adorns the polar 

 regions, where the frost scarcely gives way under 

 the heat of midsummer : and yet this plant spreads 

 over vast plains, and illumines them as if by crimson 

 snow. 



A high degree of heat is generally favorable to 

 the growth of fruit, if it be not accompanied with 

 drought. By the management of artificial heat, 

 gardeners make plants perform curious freaks. Sir 

 Thomas A. Knight caused melons and cucumbers to 

 produce all male blossoms in excessive heat, and all 

 female in a low temperature. Heat stimulates evap- 

 oration from the leaves, and therefore excites the 

 roots to absorb nourishment from the soil. With 

 the aid of light it paints the colors upon the fruit, 

 which show that the saccharine fermentation has 

 been well performed. 



