Il6 TRANSPORT OF WATER AND SOIL SOLUTES 



mother cells of sieve tubes (see page 36) and companion cells, 

 and these finally are succeeded by border parenchyma cells 

 merely. Thus it happens that at the ends of the ultimate branches 

 the vascular bundles may be represented by a single tracheid 

 merely since the border parenchyma is not morphologically 

 a part of the vascular bundle. 



Water is drawn by osmosis from the tracheids by the border 

 parenchyma cells, and from these in the same way by the palisade 

 and spongy parenchyma. From these tissues it is for the greater 

 part evaporated into the intercellular spaces, and passes thence 

 through the stomata into the external atmosphere (Fig. 59). 

 In this way some plants give off in twenty-four hours of a hot 

 summer day as much as 10 c.c. of water for each square centi- 

 meter of leaf surface; but the average rate is much lower. A 

 large birch tree has been found to give off from 300 to 400 kilo- 

 grams of water in twenty-four hours. 



Only a relatively small part of the water taken into the palisade 

 and spongy parenchyma cells is used in the manufacture of carbo- 

 hydrates. (See page 147.) 



The Power Concerned in the Ascent of Water. — The 

 force of osmosis in the root hairs and cortical cells of the young 

 roots sets up the flow of water from the soil into the tracheal 

 system; and this force communicated to the tracheal elements 

 is at times sufficient to raise water many feet; but that it does 

 not work fast enough to be the dominant force when transpiration 

 is at its -height is shown by the fact that at such times there is a 

 negative pressure in the water highways, which may be taken 

 as an indication that the effective power is in the form of a pull 

 from above. 



The tracheal tubes and tracheids are small enough for capil- 

 larity to be a significant force in water ascent and in sustaining 

 the weight of the water columns; and doubtless capillarity does 

 play an important part. But it seems from the data now avail- 

 able that capillarity under the conditions existing in plants 

 cannot lift the water fast enough, nor high enough to supply 

 the taller trees. 



