TRANSPIRATION. 273 



sap exuded, neglecting these said fluctuations, is greatest soon after the stem is cut, 

 and then becomes gradually less until finally the outflow ceases entirely with the 

 death of the stump. 



The magnitude of the pressure, and the quantity of the sap forced up by the 

 absorptive power of the cells, vary with the circumstances of the plants considered. 

 The pressure appears to be greatest in species of vine, and in the vine stem, as 

 already remarked, it will support the weight of a column of mercury 856 mm. high. 

 In the stem of the Foxglove it equals the pressure of a column of mercury 461 mm. 

 high; in the stem of the nettle the column is 354 mm.; in the poppy stem 212 mm.; 

 in the stem of a bean 159 mm.; and in the trunk of the White Mulberry tree 12 

 mm. high. In the majority of herbaceous plants this pressure is quite sufficient to 

 drive the sap from the root- tips up to the leaves and top of the stem; but this is 

 not the case with leafy trees and pines, with palms and creeping and climbing 

 plants. Although watery fluid can be raised according to the above calculation to 

 a height of 11 "6 m. by root-pressure, there is still a great distance between this level 

 and the leaves of such trees and climbing plants, which may be as much as 160 m. 

 high; and the question which presents itself is this: By what means is the sap 

 carried to the higher regions from this level to which it is raised by root-pressure? 



It may be supposed that cells are present at the various heights in the stem to 

 which the water is driven, which act in a manner similar to those of the root; i.e. 

 cells which actively absorb, whose cell- wall on one side only slightly resists filtra- 

 tion, and which therefore are able to force up the sap a little higher. The results of 

 the following experiments seem to support such a supposition. If a piece of a 

 branch be cut from the middle portion of a tree, and the lower end be peeled and 

 placed in water, sap will flow out from the upper cut surface with considerable 

 force. The same thing occurs when a leafy branch is placed in water so that its 

 leaves are submerged, while the upper cut piece of the branch projects a good way 

 out of the water. In this case the cells of the leaves must function as the absorptive 

 cells. However, even if, as is probable, parenchymatous cells are to be found at all 

 levels of the plant stem behaving exactly like the absorptive cells of the root, 

 this arrangement would scarcely suffice in all cases to carry the sap to its destination. 

 Atmospheric pressure as well as the rarefaction of the air observed in the vessels of 

 the stem during the summer have been made use of in explaining the upward 

 current of the sap, and this r61e may actually belong to these factors; but all these 

 mechanical powers are quite overshadowed by that one which has been termed by 

 botanists " Transpiration". 



TEANSPIEATION. 



By transpiration of plants we mean the act of giving off aqueous vapour to the 

 surrounding air — briefly and in plain terms, the perspiring of plants. Vapour 

 escapes from the cells of the plant which are in contact with the air, the formation 

 of these cells being specially adapted to the process of evaporation, just as it is given 



VOL.1. ' ' * 18 



