LECTURE XV. 



Conditions of transpiration— absorption of water and 

 nutritive matters by the roots of land-plants. 



The main purpose attained by means of transpiration is, as already men- 

 tioned, that large masses of water, containing very small quantities of dissolved 

 nutritive substances, are gradually carried to the organs of assimilation; these 

 nutritive substances in their turn take part in the process of assimilation, while the 

 greater portion of the water in which they were dissolved evaporates. The flow of 

 water described in the preceding lecture thus subserves nutrition in this sense. It 

 now concerns us to become acquainted more in detail with the conditions under 

 which transpiration itself takes place, and which accelerate or diminish the rapidity 

 of the flow of water, and therefore that of the supply of nutritive matters also. 

 Upon this depends, again, the rapidity with which the water enters into the ab- 

 sorbing roots ; since in a land-plant growing under normal conditions, the quantity 

 of water exhaled as vapour at the leaves is always nearly equal to that taken up by 

 the roots, except in so far as possible additional circumstances, which I shall treat 

 of in the following lecture, cause differences in this relation. 



It is very easy to establish the fact that leaves or succulent shoot-axes, from which 

 the epidermis has been taken off, as well as tubers and turnips after the peeling off 

 of their periderm, dry up very quickly. That this happens much more slowly in the 

 natural condition, is evidently due to the epidermis or cork-tissue of the periderm 

 respectively. These are permeable with difficulty, not only by water itself, but also 

 by aqueous vapour ; and this is certainly true in a still higher degree of the thick 

 bark of old tree stems. Plants produce well-developed periderm and bark, moreover, 

 only at those parts where transpiration is not to be permitted to take place. 



We are thus concerned properly with the epidermis only; this, on the one 

 hand, affords protection against the excessive evaporation of the water from the leaves 

 and young shoot- axes, and, on the other hand, is specially organised for the purpose 

 of rendering transpiration possible, and yet more, of limiting or accelerating it 

 according to circumstances. The epidermis accomplishes the first object by means 

 of the cuticle, and the waxy coatings, which, it is true, do not absolutely prevent the 

 evaporation of water from the epidermis cells, but still render it exceedingly slow. 

 The second object— the regulation of the evaporation of water — is accomplished by 



