INTRODUCTION 39 



The older tissues feed the newer and these last 

 the leaves, having larger tracheal vessels. These 

 tissues can be distinguished by putting a plant in 

 red ink, when the water-conducting vessels become 

 red. 



The root pressure at the lower extremity of the 

 plant is counterbalanced by the existence of another 

 process, or transpiration. If it were not for this the 

 plant would practically burst, filled to its utmost 

 capacity with cell-sap. 



But, by the joint processes of osmosis on the one 

 hand, enabling the entrance of water and nutrient 

 salts, and on the other of transpiration or the exit of 

 water, there is a free circulation of water in the 

 plant. The water passes out minus the nutrient salts, 

 these being retained for the elaboration of proteids, 

 etc., all the elements but carbon, and including 

 hydrogen and oxygen, being obtained by absorp- 

 tion. 



In the case of land-plants the water passes out 

 through the stomata, which occur, as a rule, on the 

 under-surface of the leaves. In water-plants trans- 

 piration is not possible. Water circulates through 

 the intercellular spaces. In floating aquatics the 

 stomata are above. 



The stomata are openings with two guard-cells, 

 one on each side, to close and open them. When 

 the guard-cells are turgid the stomata open ; when 

 they are not, as at night, they are closed. 



The vascular bundle, or main vein or midrib in a 



