242 MI V TO NO MY. 



Vidas of the Cape, and the Mesembryanthema, require a very 

 warm sunshine, and the full light of day, to unfold them- 

 selves. Bory St. Vincent forced them to open themselves, 

 by means of a powerful hght artificially produced by burning 

 glasses, (Ann. Gener. des Scien. Phys.) Many again only open 

 themselves during the hours of mid-day, as the Mesembryan- 

 thernum noctijioriim, pomeridianum, dolabriforme, and stra- 

 mineum. Some others seem to be too tender for the strong 

 stimulus of the clear light of the sun : they hence open only 

 during the evening hours, and shut again when the sun has 

 attained a certain elevation. Of this description are many 

 Silenae, CEnotherae, the Cactus grandifloriis and triangHr- 



369. 



Light displays its stimulating influence also upon plants 

 that are beginning to germinate. But necessary as it is for the 

 springing plant, it is equally injurious when it is permitted 

 to exercise too powerful an influence, especially upon very 

 fine seeds and unusually tender plants. The seed of Ferns, of 

 the Ericae, and some others of a similar nature, only germinate, 

 when, with sufficient v*farmth and moisture, they receive as 

 little of the sun's light as possible. For larger seeds, and more 

 hardy plants;, the clear light of the sun is not too powerful a 

 stimulant, 



370. 



The second common stimulus which the life of plants re- 

 quires is Heat, which is as little a material substance as light, 

 since, like it, it operates contrary to the laws of gravity, dif- 

 fuses itself in rays on all sides, and passes through the Torri- 

 cellian vacuum. Without a certain proportion of warmth, 

 no plant can spring or thrive : this principle exalts the activity 

 of plants, and when it is very powerful it exhausts thar 

 powers, as it does also those of animals. Its long absence ren- 

 ders plants as susceptible of a new excitement as animals. It 

 operates most advantageously upon the vegetable w^orld, when 

 there is a regulated change of temperature, and when thus 



