240 



PIIYTONOMY. 



366. 



Light is the principal stimulus for all living bodies, and 

 for the vegetable world in particular : without its agency no 

 plant grows, nor continues to exist. We have already con- 

 sidered (317.), the chemical attraction of light for the oxygen 

 of plants, as a very important phenomenon, and have also re- 

 marked, that the consequences of this attraction of light for 

 oxygen do not take place without the exertion of the internal 

 activity of the plant, and we must therefore admit a real con- 

 test between light and the power of the plant. The light 

 of the sun, however, is not a material substance, but a pure 

 principle of activity, because, without any perceptible loss of 

 time, it traverses the immeasurable extent of heaven, — be- 

 cause it penetrates even through space that is deprived of 

 air, — and because it every where acts in opposition to the 

 laws of gravity. It operates, therefore, on plants, simply as 

 a vital stimulant, and excites all the functions of the plant to 

 such a degree, that all its secretions are more powerfully per- 

 formed ; on which account even corn is not so nutritious in 

 wet seasons, when there have been many tempestuous days. 

 It is hence that branches and leaves push towards the light. 

 We hence also remark, that plants, which have long been 

 withdrawn from the light, suffer an exhaustion of their powers, 

 become white, and wither, when light has suddenly been ad- 

 mitted to them. Nay, in one instance, (Cludius in der Gar- 

 ten Zeitung, b. i. s. 386.), after a long removal of light, an 

 evident quaking of the plant was observed, as if it had been 

 agitated by the shivering of a fever, and even a leaf of the 

 D ionaa musdpula, which has been torn off, makes repeated 

 attempts to open itself to the light. These attempts are seen 

 in undulating movements of the marginal cilia, during which 

 the surface of the leaf is successively opened and shut, until 

 it entirely opens, and thereby loses its excitability ; (Nuttal, 

 Genera of North American Plants). It seems evep to be es- 

 tablished by ancient and modern observations, that the light 

 of the moon has some influence on plants ; (Wilson, Uber 

 der Einfluss der Klima's auf Pflantzen und Thicre, s. 16,) 



