THE MATERIAL OUTGO OF PLANTS 341 



3. THE MOVEMENT OF WATER 



Transpiration stream. — In the two foregoing sections it has appeared 

 clearly that the region where water enters a plant and the region whence 

 it leaves are rarely identical, but that these parts arc more or less widely 

 separated. There must be, therefore, movement of wakr through the 

 body. Small quantities of water are used in the body for saturating 

 new-made materials and parts, and for food making by green plants. 

 Somewhat larger quantities arc exuded by guttation, bleeding, or secre- 

 tion. But the dominant cause of movement is to be found in evaporation, 

 for the amount thus leaving the body is often many times greater than 

 all other quantities combined. So considerable is it that the How through 

 the body is figuratively known as the transpiration stream. 



Transfer in small plants. — In the smaller land plants, whose bodies 

 are composed of living cells throughout, as in many liverworts and 

 mosses, the water has to travel but a short distance, and the movement 

 can be osmotic only. Evaporation at an exposed surface concentrates 

 the solutions in those cells, thereby reducing the internal pressure of the 

 water, which moves from an adjacent cell to reestablish equilibrium, 

 and so the disturbance soon reaches the surface cells in contact with free 

 water, which enters the plant. 



Origin of a conducting system. — We might infer that these osmotic 

 movements are too slow to afford a proper supply to larger plants, be- 

 cause, as an actual fact, they are in operation for only relatively short 

 distances; the larger the plant and the more necessary a large supply of 

 water, the more perfect and extensive becomes the special system of 

 tissues for conducting water by avoiding osmotic transfer. This is 

 especially striking when one follows the development of such a plant as 

 a sunflower from the embryo, a stage when there is no water-conducting 

 tissue, to maturity, observing how the extent and amount of this tissue 

 increases as the foliage develops and so increases the evaporating sur- 

 face. It may be assumed that somewhat similar has been the history of 

 the evolution of land plants. As the early aquatics became more and 

 more exposed to evaporation, there probably came about the develop- 

 ment of structures which limit the water loss, and simultaneously the 

 development of the water-conducting strands, which greatly facilitate 

 water movement. 



Elongation of cells. — Presumably one of the simplest expedients to 

 accelerate movement is to reduce the number of membranes which the 



