PATH OF WATER 75 
(e.g. in the tracheary tissues) is sufficient to pull the 
water up from the lowest roots. Other investigators 
have suggested that some of the living parenchyma cells 
which accompany all water-conducting tracheids and 
tracheae are concerned in the lifting of the water (or 
ascent of sap as it is often called). 
111. Path of the Water. ‘This is chiefly in the cavities 
(lumina) of the tracheary tissue. It is also not to be 
denied that the water will pass upward slowly from the 
roots, passing from cell to cell in the parenchyma by 
osmosis, for the tissues above ground have more con- 
centrated solutions, and so bring about osmosis from the 
root cells with their less concentrated solutions. This is, 
however, not sufficient to supply an ordinary plant. 
Within the tracheary tissue, the lumen contains not only 
water but some bubbles of air, past which the water flows 
in a thin film next to the cell wall. In trees the central 
wood after a number of years suffers deposition of resins 
or other insoluble substances within the cell cavities and 
possibly walls as well, so that water conduction is no 
longer possible. Such wood is often different in color 
and is called heart wood and contains no living cells. 
The unchanged wood around it, the sap wood, contains 
dead water-conducting tracheary tissue, dead fibrous 
tissue and living wood parenchyma. 
112. The evaporation of water from the leaves and 
stems is often given the name transpiration. It is an 
unavoidable loss since the plant must have openings, 
the stomata, through the epidermis, for the purpose of gas 
exchange and when these are open the loss of water can- 
not be prevented. The thickening of the cuticle in 
plants of dry regions, the depression of stomata in the 
pits to provide dead air spaces outside, the formation of 
thick layers of hairs, etc., all indicate that it is not to the 
