THE STRUCTURE AND WORK OF THE STEM 107 



in water. White of egg, for example, is very slightly soluble, but can 

 be rendered insoluble by heating it so that it coagulates. Insoluble 

 proteids are digested within the plant ; how and where is but slightly 

 understood. In a plant, soluble proteids pass down the sieve tubes 

 in the bast and then may be stored in the bast or medullary rays 

 of the wood in an insoluble form, or they may pass into the fruit or 

 seeds of a plant, and be stored there. 



What forces Water up the Stem. We have seen that the process of 

 osmosis is responsible for taking in soil water, and that the enormous ab- 

 sorbing surface exposed by the root hairs 

 makes possible the absorption of a large 

 amount of water. Frequently this is more 

 than the weight of the plant in every twenty- 

 four hours. 



Experiments have been made which show 

 that at certain times in the year this water 

 is in some way forced up the tiny tubes of 

 the stem. During the spring season, in 

 young and rapidly growing trees, water has 

 been proved to rise to a height of nearly 

 ninety feet. The force that causes this rise of 

 irntt-r in stems is known as root pressure. 



But root pressure alone cannot account 

 for the rise of sap (water containing materials 

 taken out of the soil) to a height of several 

 hundred feet, as in the stems of the California 

 big trees. Other forces must play a part here. 

 One way in which the rise of water can be 

 partly accounted for is in the fact that capil- 

 lary attraction may help in part. If you place 

 in a glass containing red or other colored fluid three or four tubes of differ- 

 ent inside diameter, the fluid will be found to rise very much higher in the 

 tubes having a smaller diameter. This is caused by capillarity or capil- 

 lary attraction. When we consider that the tubes in the stem are very much 

 smaller than any we can make out of glass, it can be seen that water 

 might rise in the stem to some height in tubes of microscopic diameter. 



The greatest factor, however, is one which will be more fully explained 

 when we study the work of the leaf. Leaves pass off an immense quan- 

 tity of water by evaporating it in the form of vapor. This evaporation 

 seems to result in a kind of suction on the column of water in the stem. 

 In the fall, after the leaves have gone, much less water is taken in by 

 roots, showing that an intimate relation exists between the leaves and 

 the root. 



Diagram to show the areas in 

 a plant through which raw 

 food materials pass up the 

 stem and food materials 

 pass down. (After Stevens.) 



