PROBLEM 2 



The Part Ste?ns a?hi Roots Flay hi Making Food 159 



rises in each xylem tube. You may have 

 seen water rising several feet in a thistle 

 tube in the experiment showing diffusion 

 through a membrane. But our problem 

 now is to explain the rise of water not 

 three or four feet but a hundred feet or 

 three hundred feet in tall trees. How 

 does it get to the leaves at the very top 

 of a tree? 



It has been known for a long time that 

 if a very narrow tube is dipped into 

 water the water will rise in the tube. 

 The narrower the tube the higher the 

 water will rise. This is called capillary 

 action; the narrow tube is called a capil- 

 lary tube. Water may rise in each xylem 

 tube by capillary action. This tube, 

 though extremely long, is the finest kind 

 of capillary tube for it is microscopic in 

 diameter. It is the width of only one 

 cell. Since it is so fine a tube, water read- 

 ily rises in it for some distance. Do Ex- 

 ercise 13 to see capillary action. 



Another force that causes water to 

 rise in xylem tubes was discovered about 

 thirty years ago. It was discovered that 

 water in a capillary tube stays together 

 in a column as if it were a wire. If some 

 kind of pull is given at the upper end of 

 such a column the whole column moves 

 up. 



The pull on the water in the xylem 

 tubes. Take another look at Fig. 185, page 

 158. What the picture does not show 

 is that the veins with their vascular tissue 

 divide into such tiny branches that but 

 very few cells lie between any two of 

 the tiniest veins. Water coming up 

 through the xylem tubes constantly dif- 

 fuses from the tubes to the neighboring 

 cells. All the leaf cells, including the 

 epidermal cells, hokl large quantities of 



water both in the protoplasm and in the 

 cell wall. Now if the concentration of 

 water in the cells is greater than it is in 

 the surrounding atmosphere, the water 

 passes off into the air. We say the water 

 evaporates. 



Very little of the water leaves directly 

 from the epidermal cells. Most of it goes 

 out from the air spaces through the sto- 

 mata. Every air space within the leaf is 

 surrounded by cells from which water 

 molecules are separating and diffusing as 

 water vapor. From the air spaces this 

 water vapor diffuses outward through 

 the stomata. Even when the stomata are 

 said to be completely closed there is 

 still enough of an opening for water 

 molecules to pass through. And at no 

 time would all the stomata of a plant be 

 completely closed. When water leaves 

 the cells and diifuses into the surround- 

 ing air we call the process transpiration. 

 If you have the equipment you will find 

 Exercises 14, 15, and 16 worth while. 

 Transpiration thus decreases the concen- 

 tration of water in the leaf cells near the 

 upper end of the water column in the 

 xvlem tubes. Water then diffuses from 

 the tubes into these leaf cells and thus 

 pulls on the column of water. Because 

 it is transpiration that decreases the con- 

 centration in the leaf cells this pull has 

 been called the lifting power of tran- 

 spiration. 



Transpiration important to the farmer. 

 A single com plant may lose three or 

 four quarts of water on a hot day. A 

 birch tree with about 200,000 leaves loses 

 as much as 350 quarts on a hot dry day 

 in summer. These large amounts of water 

 vapor in the atmosphere condense in 

 time and come down as rain. Can you 



