170 BOTANY. 



mainly due to the increased heat and dryness which are 

 common accompaniments of the increase of light.* 



221. In enumerating the internal conditions one general 

 one must not be forgotten, which is, that the water in plant- 

 cells contains many substances in solution, and consequently 

 evaporates less rapidly than pure water, in accordance with 

 well-known physical laws. Moreover, the attraction of the 

 molecules of the cell-walls for the water layers counteracts, 

 to a considerable extent, the tendency to evaporation ; and 

 in the same manner, even to a greater extent, the water is 

 prevented from passing off by the "imbibition power" of 

 protoplasm. It is, in fact, impossible to deprive cellulose 

 and protoplasm of their intermolecular water in dry air at 

 ordinary temperatures. 



In all the aerial parts of higher plants the epidermis 

 offers more or less resistance to the escape of the water of the 

 underlying tissues. This is mainly accomplished by the 

 thick and cuticularized outer wall of the epidermal layer ; in 

 many cases, especially in plants growing naturally in very 

 dry regions, the epidermis consists of several layers of cells, 

 which offer still more resistance to evaporation by being 

 themselves filled with moist air only. Among the lower 

 plants, the single reproductive cells (spores) are guarded 

 against the loss of water by having their walls greatly thick- 

 ened and cuticularized. Even in the lowest plants, the Slime 

 Moulds (Myxomycetes), the naked masses of protoplasm, 

 when placed in dry air, will contract into rounded masses, 

 which then become covered with a somewhat impervious 

 envelope (paragraph 23, c : page 21). 



222. The stomata of the green and succulent parts of 

 higher plants control to a great extent the amount and 

 rapidity of their exhalation. In leaves, for example, where, 

 on account of its cuticularization, there can be but little 

 evaporation through the epidermis, it is dependent upon the 



* I am aware that some experiments made with plants in saturated 

 and in dry air appear to show that in direct sunlight there is a rapid 

 evaporation. I cannot, however, regard these experiments as con. 

 elusive. 



