MOVEMENT OF WATER IN PLANTS. 
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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period of vegetation the quantity of water which has been withdrawn by tran- 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water withdrawn from the leaves is replaced in this way.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 
