260 BIOLOGY. 



of the sun. This turning must, nevertheless, have one 

 arid the same kind of cause with the growing towards 

 light ; it also is only a conatus or effort unto growth. 

 The upper leaf-cells, being illuminated by the sun, be- 

 come lighter, and are therefore directed at once, like 

 the apices of the branches, toward the influence of light. 

 The cells that stand perpendicularly upon the surface of 

 the leaf are to be regarded as branches conjoined by 

 growth. 



1374. The sleep of plants depends also upon the same 

 influence of the light. The upper leaf-cells sleep during 

 the night, while the lower cells, especially those of the 

 petiole, fill and consequently bend the latter upwards. 

 The sleep of the flower must have the same cause. As 

 likewise the alternating motion of many leaves, as in the 

 Mimosse. 



1375. The motion also performed by the staminal 

 filaments towards the pistil must finally depend upon 

 this unequal replenishment of the external and internal 

 cells. 



1376. The coloration of the parts of plants is a result 

 of the decomposition of the starch-granules in the cells 

 by the agency of light. 



1377. The operation of the heat is more intelligible 

 than any other. That which is to move and separate 

 itself, must have a certain degree of extension, or must 

 be fluid, namely, aquiform. In a cold temperature the 

 upper saps, not becoming warmer than the lower, are 

 consequently not lighter, and on that account also do 

 not ascend upwards. The mortal freezing of trees de- 

 scends from above downwards. In other respects plants 

 have, like animals, a self-inherent, though very feeble, 

 process of heat. Germination proves this, in cases where 

 many seeds lie upon each other. 



1378. The air acts also mechanically and physically 

 upon plants, by causing motion of the solid parts and 

 by promoting evaporation. Electricity is without doubt 

 active in the spring of the year, and evokes the antago- 

 nism between the fabric of the stem and root. 



