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 



