122 



HUTCHINSON'S POPULAR BOTANY 



passage as the water which passes through a pump. The wind which rocks 

 the trees and plants to and fro assists in this process, and the leaves also 

 assist, though in quite a different manner. The latter, indeed, are the 

 busiest organs of the plant, as we shall see in the next chapter, when we 

 shall consider their wonderful structure and functions. 



The rate at which the watery sap courses up the stem may be gathered 

 from Kingsley's vivid description of the Liantesse (Schnella excisci), a West 

 Indian Water-vine, whose singular stem, hanging in loops twenty feet high, 



he likens to a chain-cable between two 

 flexible iron bars. At one of these 

 loops, "about as thick as your arm," 

 writes the Canon, ",your companion, if 

 you have a forester with you, will 

 spring joyfully. With a few blows of 

 his cutlass he will sever it as high up 

 as he can reach, and again below, some 

 three feet down; and while you are 

 wondering at this seemingly wanton 

 destruction, he lifts the bar on high, 

 throws his head back, and pours down 

 his thirsty throat a pint or more of pure 

 cold water. This hidden treasure is, 

 strange as it may seem, the ascending 

 sap, or rather the ascending pure rain- 

 water which has been taken up by the 

 roots, and is hurrying aloft to be elabo- 

 rated into sap, and leaf, and flower, and 

 fruit, and fresh tissue for the very stem 

 up which it originally climbed ; and 

 therefore it is that the woodman cuts 

 the Water-vine through first at the top 

 of the piece which he wants, and not at 

 the bottom ; for so rapid is the ascent 

 of the sap that if he cut the stem 



below, the water would have all fled upwards before he could have cut it off 

 above " (At Last, p. 159). 



The " pure rain-water " mentioned by Kingsley is not really pure, for 

 it contains mineral elements from the soil dissolved in it. The plant 

 requires this mineral matter to mix with the gases taken in from the 

 atmosphere, that all may be elaborated into sap in the leaves. But the 

 percentage of mineral constituents is very low, so that a vast volume of 

 water must be given off as vapour through the stomata, and this increases 

 the pulling action which helps the upward flow. 

 So much for the ascending sap. 



FIG. 156. AN AQUATIC FLY-TRAP 

 (Aldrovanda). 



A small rootless plant that Darwin called "a minia- 

 ture aquatic Dionsea." A single whorl of leaves is 

 here shown, together with an enlargement of a single 

 leaf. EUROPE, INDIA, AUSTRALIA. 



