132 PLANT PHYSIOLOGY 



the greater the loss of water in proportion to the absorp- 

 tion the lowfer the pressure, local and general. Copious 

 absorption is dependent upon the presence in the cells of a 

 large amount of soluble and osmotically active substance 

 and upon the presence outside of a large amount of water. 

 Small loss of water is dependent upon small surface, upon 

 the impermeability of the walls of the superficial cells, and 

 upon low protoplasmic activities in them. These condi- 

 tions are met especially in spring, but also to a limited extent 

 during and immediately after rain in summer. In spring, 

 water is taken up in quantity from the moisture-laden soil 

 by the dense cell-sap of the root cells; from the still bare 

 branches and the unopened buds water is given off only in 

 very small amount;* sap-pressure develops in consequence. 

 If a plant in this condition has been so recently trimmed 

 or pruned that the wounds are not yet closed, or if new 

 wounds are opened, we shall have the familiar phenomena 

 of "bleeding" and of sap-flow. The name "bleeding" or 

 "weeping" is given to wholly useless if not injurious ex- 

 hibitions of the phenomena, employed by the farmer in 

 northern North America when he "taps" his maple trees 

 in spring to secure the highly prized maple syrup and maple 

 sugar. WTiat flows from the plant, whether in bleeding or 

 in the run of sap after tapping, is the water expressed into 

 the wood elements by the living cells bordering upon them. 

 This sap, flowing out under pressure, is a solution contain- 

 ing various organic compounds — in maple chiefly sugars — 

 and mineral salts. The presence of mineral salts in the sap, 

 and their accumulation in the evaporating pans employed in 

 sugar-making, are due to their being taken up, a little at a 

 time, by the plant in the water absorbed from the soil. The 

 amount and the kinds of salts present in the sap will vary 

 with the nature of the soil and with the kind of plant, for 

 the reasons which we have above considered (p. 112). The 



* It is not a question of surface merely, however, for in evergreens, 

 although the surface is not materially less in winter and spring than in 

 summer, the amount of water given off during winter and spring is much 

 less than in summer. The lessened activities of the protoplasm in leaves 

 and branches, and the decreased evaporation at the lower temperatures, 

 account for this. 



