226 LECTURE XIV. 



the purpose last named, in one period of vegetation, may be many times larger 

 than the volume of the whole plant itself. We have thus to do here with an 

 exceedingly energetic function; and, disregarding for the time being all other 

 movements of water in the plant, we will concern ourselves in the first place 

 exclusively with the phenomenon last indicated, which is called into play by 

 the evaporation of water at the surfaces of the leaves, and which we may 

 best distinguish as the ascent of water in transpiring land-plants. When the 

 phenomena which here take place and co-operate have been made clear, it 

 will be easy to understand deductively the movements of water in other plants 

 also. 



In the first place it will be advantageous to point out the purpose for which the 

 flow of water is brought about by the transpiration of the leaves. The green leaves 

 containing chlorophyll are, as has been already mentioned several times, the organs 

 of assimilation of the plant ; and it is in their cells that the carbon dioxide of the air 

 is dfecomposed, and employed for the production of carbonaceous vegetable substance. 

 Water also is necessary for the formation of this substance ; and it is known, and will 

 be shown in detail later on, that a series of salts which the roots take up from the 

 earth are absolutely necessary in addition, if the process of assimilation in the cells 

 containing chlorophyll is to take place. These salts (especially sulphates and 

 phosphates of calcium and magnesium, potassium nitrate, and salts of iron) must 

 therefore be conveyed to the green leaves for the purpose of producing organic sub- 

 stance. This is accomplished as follows : an exceedingly dilute solution of these salts, 

 which may be compared at once to ordinary drinking water, flows from the roots 

 through the stem into the leaves. Considering the small amount of salts contained in 

 the ascending water, however, only an extremely small quantity of the nutritive sub- 

 stances would be conveyed into the assimilating cells if the matter ended with the 

 mere flowing in. The assimilating leaves, however, are induced by the warmtii 

 of the surrounding air, and especially by the rays of light, to let the water which 

 has streamed into them escape in the form of vapour; this enables a fresh 

 quantity of water charged with nutritive matter to flow up to them from the 

 roots. Thus, by means of evaporation or transpiration, a continuous flow into the 

 organs of assimilation is rendered possible : as the water evaporates from these 

 organs, the salts brought by it from the soil remain behind in the assimilating leaf-cells, 

 and take part in the chemical processes of nutrition. This obvious explanation of the 

 ascent of the water, so far as it is caused by transpiration, has hitlierto been almost 

 entirely misunderstood in a remarkable manner ; and short-sighted people have even 

 maintained that the transpiration of land-plants is a faulty arrangement, which has to 

 be compensated by means of the currents of water. As a matter of fact, however, 

 the whole organisation of a land-plant is only intelligible if one keeps in view the 

 purpose of the water current indicated. The envelopment of the stem with an 

 epidermis which possesses few stomata and hinders evaporation, or, in woody 

 plants, even with a thick periderm, and in large trees with bark, has essentially the 

 object of protecting the nutritive water ascending in the stem against evaporation. 

 The small thickness and large area of the leaves, combined with the existence of 

 the stomata which pierce the epidermis in millions, and which may be closed or 

 open according to circumstances, are likewise only intelligible when it is known 



