PHYSIOLOGY 185 



If, instead of the eifects of the pressure, the volume of water exuded each hour 

 be observed, the remarkable fact will be demonstrated that the roots regularly 

 discharge more water at certain hours than at others (Periodicity of Root- 

 Pueskiike. 



When it was shown that the roots were capable of exercising so great a 

 pressure, it was at first believed that the ascent of the sap to the tops of the 

 highest trees was due to root-pressure. This, however, would be impossible in 

 view of the following considerations. The volume of water supplied by root- 

 pressure is not sufficient to satisfy the quantity given off by evaporation. On the 

 contrary, by moderately vigorous transpiration, such as takes place on a summer 

 day, the root-pressure is of a negative character. Thus, if an actively evaporating 

 plant be cut off near the root, no outflow of water will take place. On the other 

 hand, the stump will energetically draw in water supplied to it ; and not until it 

 lias become saturated does the force of the root-pressure make itself apparent. In 

 ]->lants growing under natural conditions, the root-pressure is only effective on 

 damp, cool days, or at nights, when the transpiration is greatly diminished. In 

 spring, when the roots are beginning their activity, the conditions are most favour- 

 able, the wood is full of water, and the transpiring leaves are not yet unfolded. 

 When the wood is injured, "sap" is exuded in drops from the vessels and tracheids. 



The so-called bleeding from wounds or cut stems is chiefly due 

 to root-pressure, but it is also augmented by the pressure exerted by 

 the living cells of the wood (wood parenchyma, medullary rays). 

 The out-flowing sap often contains, in addition to numerous 



SALTS, CONSIDERABLE QUANTITIES OF ORGANIC SUBSTANCES (dissolved 



albuminous matter, asparagin, acids, and especially carbohydrates). 



The amount of saccharine matter in the sap of some plants is so great that 

 sugar may be profitably derived from it. The sap of the North American sugar 

 maple, for example, contains from 2 to 3 per cent of sugar, and a single tree will 

 yield 2-3 kilos. The sap of certain plants is also fermented and used as an 

 intoxicating drink (palm wine, pulque, a Mexican beverage made from the sap of 

 the Agave, etc.) 



The bleeding which takes place on warm, sunny winter days from wounds or 

 borings in trees is not due to root-pressure, but to- purely physical causes. It is 

 brought about by the expansion of the air-bubbles in the tracheal elements of the 

 wood, and may be artificially produced at any time in winter by warming a freshly- 

 cut piece of wood ; when the wood is allowed to cool, the air contracts and the 

 water in contact with the cut surface will be again absorbed. 



II. The Water-conduction in Plants. — In living plant-tissues 

 the cells of which require more or less water for their growth, for the 

 maintenance or augmentation of their turgidity, and to supply the 

 water lost by transpiration, there is a constant transfer of water 

 from one cell to another. This transfer between the adjacent cells 

 takes place much too slowly to equalise the great amount of water 

 lost by evaporation from the foliage of a tall tree. In order to 



TRANSFER THE WATER, QUICKLY AND IN LARGER QUANTITIES, FROM 

 THE ROOTS TO THE LEAVES, PLANTS MAKE USE, NOT OF THE LIVING 

 PARENCHYMA, BUT OF THE WOODY PORTION OF THE VASCULAR 



