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 are more or less widely 
separated. There must be, therefore, movement of water 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 are 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 flow 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 
