170 



GREEN PLANTS AS FOOD MAKERS 



the stem grows older. Sieve tubes or phloem are in the outer part 



and xylem or water-bearing tubes in the inner part of the bundle. 



What causes water to rise in a stem. We have already seen that 



osmosis is responsible for getting water inside the root, and that 



the pressure exerted by this 

 water (root pressure) is fre- 

 quently capable of forcing 

 fluids a considerable dis- 

 tance up a living stem 

 sometimes 20 or 30 feet in 

 height. But during most 

 of the year root pressure 

 plays a very unimportant 

 part in this phenomenon. 

 It has been found that in 

 the very tiny tubes, such 

 as we find in wood, the 

 rising column of water is 

 held together by the force 

 of cohesion. A core of 

 water in tubes ^ °f an 



"feoct, __ 

 distributed 

 bv tubes 

 to needV 

 plant parts- 



soil sv&ter 



Yiasa part 



mthe 



waking €f 

 foocC 1T1 

 the leaf 



soil Vater- 

 travels 



tubefto 

 leaves 



, Soil 



W*-vater 



^ enters 



''^ plants 



- - through 



rootr 



Yiacirs- 



inch in diameter will with- 

 stand a pull of over 4600 

 pounds to the square inch, 

 so it is likely that this force 

 is the strongest factor in 

 raising water in the tubes 

 of tall trees. Also a very 

 large amount of water is 

 evaporated every day, a 



Fluids pass up and down in plants through tubes, tree of average size Using 

 Raw materials pass upward into the leaves. The & & 



food manufactured in the leaves passes downward from 75 to 100 gallons of 

 to the roots for storage, and to the other parts of the , ., r ,. i 



plant which need it. water daily, most of which 



passes out through the 

 stomata. This evaporation may cause a pull on the volume of 

 water in the fibrovascular bundles and probably is another im- 

 portant factor in the rise of fluids in stems. 



