PLANT PHYSIOLOGY 127 



The method by which the sap is carried up the stem is only 

 partially understood; there are several factors concerned. One 

 factor is osmosis. The water from the soil is absorbed by the 

 root hairs principally through the physical force of osmosis, a 

 force which is capable of causing some substances to pass, even 

 against resistance, through the thin-walled root hairs, while 

 others are rejected. An osmotic pressure is thus produced in 

 the root, due to the absorption of liquids from the soil, and this 

 forces a current up through' the stem. 



A second factor is the absorptive power of protoplasm. Liv- 

 ing protoplasm has a strong avidity for water and absorbs it 

 until it is saturated. If a plant were in absolute equilibrium, 

 each bit of protoplasm would absorb all the water that it could 

 obtain and a condition of rest would soon appear. If, however, 

 a cell loses any of its liquid, it will have at once a stronger 

 demand for water than before, and will tend to draw it away 

 from neighboring cells that are more nearly saturated. Hence 

 in a plant there will be a constant flow of water from saturated 

 parts to those less saturated. In an ordinary green plant there 

 are several processes that use up the water, all of them especially 

 active in the leaves and growing buds at the top of the plant. 

 These are as follows: 



1. Water is being used in the leaves to manufacture starch. 



2. New protoplasm is being made in the leaves and in the 

 growing buds, and this new protoplasm demands water. 



3. Water constantly evaporates from the leaves through the 

 stomata (transpiration). The extent of this evaporation varies 

 greatly with the warmth and dryness of the air and also with 

 the extent to which the stomata are opened. When there is 

 abundance of water in the plant, the stomata are widely open 

 and evaporation is rapid; but when the water is insufficient 

 these pores partly close and evaporation is checked. On a warm 

 day when the air is dry the evaporation is increased, but in a 

 cool damp atmosphere it is lessened. 



A third factor is capillarity; this is the same force that 



