MOVEMENT OF WATER' IN PLANTS, 675 



superficial development; when the leaves are also delicate, as in most plants with 

 a rapid growth, a very considerable portion of the water of their cell-sap is 

 removed by transpiration within a short time, so that in the course of a single 

 period of vegetation the quantity of water which has been withdrawn by tran- 

 spiration may exceed many times the weight and volume of the plant itself. It is 

 easy to understand that this is possible only when the loss is compensated by the 

 absorption of corresponding quantities of water through the roots, and that the 

 water withdrawn from the leaves is replaced in this way. As long as the tissue 

 of plants in which transpiration takes place remains turgid, the supply must nearly 

 equal the loss; so long therefore as transpiration proceeds continuously from the 

 leaves or other surfaces, a constant current of water exists from the roots to the 

 leaves. When transpiration ceases, as in very moist air, or when the leaves are 

 wetted by dew or rain or after the falling of the leaves, the current of water also 

 ceases as soon as the tissues which have become somewhat flaccid are again tur- 

 gescent. Since transpiration 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 transpiration has, as will be seen, no 

 immediate connection with the processes of growth and nutrition ; 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 transpiration, and with the 

 latter 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 transpiration 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 which have a compact wood, one main current passes 

 through the root and stem, dividing in the 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 observation, but also from the fact that the formation of 

 wood is the more considerable, the greater the evaporation and the stronger 

 the current of water in a plant. In submerged and underground parts of 

 plants from which no transpiration takes place the xylem remains entirely or 

 nearly unlignified; in Dicotyledons and Conifers, where the transpiring 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 



x x 2 



