The Nature of Plants 



sap 



tion 



upward the sap that has entered before. The mixing of 

 two liquids through a membrane which separates them 

 is called " osmosis," and the pressure that produces 

 it is called " osmotic pressure," or in plants " root 

 pressure." 



The plant stem. The stem, or in trees the trunk, The rise of 

 connects the water- and food-gathering roots with the 

 food factories, the leaves. Up through the channels 

 in the stem, and into the veins of the leaves, the 

 crude sap travels, carrying the dissolved mineral 

 matter taken from the soil by the roots. 



The larger part of the water in the sap that enters 

 the leaves evaporates from them. This giving off of 

 water by a plant is called " transpiration" (Fig. 7). Sap Transpira- 

 is drawn up to the leaves 

 to replace the water that 

 is lost. If sap cannot 

 be drawn up as fast as 

 it is evaporated, the 

 leaves wilt. 



As water is tran- 

 spired, the mineral mat- 

 ter that the sap con- 

 tained is left behind to 

 be made into plant 

 food. Some of the 

 water does not evapo- 

 rate but unites with 

 other materials in the 

 leaf for the manufac- FlG - 8 - How sap flows in a tree : A > outer 



bark; B, inner bark; C, growing layer 

 ture Of plant food. (cambium); D, sapwood; E, heartwood. 



Material 

 retained 



ABC D 



D CBA 



