MOVEMENT OF WATER IN PLANTS. /^ng 



takes place remains turgid, the addition must nearly equal the loss by evaporation ; 

 so long therefore as evaporation proceeds continuously from the leaves or other 

 surfaces, a constant current of water exists from the roots to the leaves. When 

 evaporation ceases, as in very moist air when the leaves are wetted by dew or rain 

 or after the falHng of the leaves, the current of water also ceases as soon as the 

 tissues which have become somewhat flaccid are again turgescent. Since evapor- 

 ation is accelerated by a high temperature of the air, by its dryness, and above all 

 by sunshine, and as these conditions are constantly changing, the rapidity of the 

 current of water is also subject to continual change. 



The current of water occasioned by evaporation has, as will be seen, no 

 immediate connection with the processes of growth and nutriment ; the horse- 

 chestnut and other trees and shrubs which put out in spring only a definite number 

 of leaves, and during the summer do not any further increase their foliage, transpire 

 the most rapidly during this time ; and at this time also the current of water is 

 most considerable in them. In winter both growth and evaporation, and with this 

 last the amount of water also in the tissues, remain stationary ; when the buds are 

 put out, the water is first of all only set in motion to the extent required by the 

 increase of the growing organs ; but as the development of the organs increases their 

 surface, the amount of evaporation again rises, and the current begins afresh. 



While the movement of water required for purposes of growth and nutrition 

 must take place in the most different forms of tissue — in the parenchyma and even in 

 the primary meristem of buds and of the apices of roots — it is nevertheless certain 

 that the current of water caused by evaporation passes exclusively through the woody 

 portion of the fibro-vascular bundles ; all the rest of the tissue may be destroyed 

 at any place without the current of water ceasing, if only the wood remains entire. 

 In Conifers and Dicotyledons a strong current passes through the root and stem, 

 dividing in die branches and leaves into constantly narrower channels ; while in 

 Ferns and Monocotyledons the current of water passes, even in the primary stem, 

 through isolated narrower channels corresponding to the course of the isolated 

 woody bundles. That the lignified elements of the xylem of the fibro-vascular 

 bundles determine the channel of the current, is seen not only from direct observa- 

 tion, but also from the fact that the formation of wood proceeds the more rapidly 

 the more considerable is the evaporation and the stronger the current of water in a 

 plant. In submerged and underground parts of plants from which no evaporation 

 takes place the xylem remains entirely or nearly unlignified ; in Dicotyledons and 

 Conifers, where the evaporating surface increases with age, the channel taken by 

 the current is also annually widened by the increase of the wood. The crown of 

 leaves of palm-trees remains after a certain time of nearly the same size, and the 

 stem and the channels of the current (woody bundles) which traverse it consequently 

 retain their diameter unchanged. 



The movements of water caused by growth as well as those induced by 

 evaporation have this in common, that their direction is towards the places wliere 

 they are required. If the growth or the evaporation begins at a certain time at 

 a definite spot, the nearest portions of the tissue give up their water first of all, 

 then the more distant ones, until at length the organs at the greatest distance, 

 generally the roots, are compelled to absorb water from without. The movement 



