[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 325 



When the leaves have begun to develop, the "sap pressure" becomes 

 negative, sap ceases to flow from the tap-holes, and (with experimental 

 equipment) water is drawn into the tree instead of being forced out. 



During the maple sap season, most of the sap and sugar flows out 

 before noon of any one day. The total yield of sugar from different trees 

 is roughly proportional to their summer leaf areas. When three taps were 

 made in a tree, one at the top of the root, another at a height of four 

 feet, and a third just below the branches, one-half of the yield came from 

 t'he tap four feet from the ground, one-fifth from tlie root tap, and the 

 remainder from the upper tap. These are some of the more interesting 

 facts about the flow of maple sap. Experimental data seem to indicate 

 that a satisfactory explanation of all the phenomena will invoh'e not only 

 root pressure and gas pressure, but also other internal relations now 

 unknown. 



Dead root systems. If water is pulled upward in stems through xvlem 

 cells that are dead, what will be the effect of killing the roots without 

 injuring the stem? An answer to this question mav be obtained bv killing 

 the roots of plants with hot water. Of course the living membranes of the 

 roots of these plants will be destroved and osmotic movement of water 

 ended. Experiments have indicated that not onlv mav the plants continue 

 to live for a week or two but the transpiration rate increases and in 

 some plants becomes several times as great if the soil is wet. Under 

 what conditions and in how many ways are living root systems superior 

 to dead root systems? 



Absorption of inorganic salts. Not only does water enter plants from 

 the soil, but inorganic salts also enter from the soil solution. When plants 

 grow freely suspended in water, as the algae in the microcosm described 

 in Chapter XX, soluble salts in the water mav diffuse into any cell of the 

 plant. In rooted plants the salts enter mainly through the root svstem. 

 A smaller amount may enter through rhizomes, or even through leaves 

 and young stems that have been covered by a film of salt-containing 

 water. This is particularly true of leaves of Spanish moss, pineapple, 

 and other plants of the bromelia family. The peculiar leaves of the 

 pitcher plant exemplify another condition in which salts may diffuse 

 directly into the leaves. Perhaps many other leaves with cup-like bases 

 in which water collects during rains similarly absorb a small amount of 

 salts directly. 



Salts in solution may diffuse directly into the root tip and root hairs, or 

 the outer layer of pectic compounds on the root hairs may be in such close 



