BUDS AND STEMS 



113 



The food material may be passed in a soluble form until it comes to a 

 place where food storage is to take place, then it can be transformed to an 

 insoluble form (starch, for example) ; later, when needed by the plant in 

 growth, it may again be transformed and sent in a soluble form through 

 the stem to the place where it will be used. The processes by which starch 

 is mada soluble in the stem in the form of sugar and then changed back 

 again to starch are but little understood. 



Building of Proteids. — Another very important food sub- 

 stance stored in the stem is protdd. Of the building of proteid 

 little is known. We know it is an extremely complex chemical 

 substance which is made in plants from compounds containing 

 nitrogen, the nitrates and compounds of ammonia received 

 through the roots from the organic matter contained in the soil, 

 in combination with sugars or starches of the plant body. 



Some forms of proteid substance are soluble and others insoluble in water. 

 White of egg, for example, is slightly soluble but can be rendered insoluble 

 by heating it so that it coagulates. 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. 



What forces Water up the Stem. — We 

 have seen that the process of osmosis is responsi- 

 ble for taking in soil water, 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 fibro vascular 

 bundles. It can be shown to rise a few inches 

 in some stems by a laboratory experiment. This 

 is best seen in the dahlia 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 water in stems is known as root pressure. 



But root pressure alone cannot account for the rise of water (as in the 

 stems of the big trees of California) to a height of several hundred feet. 

 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 capillary attraction may help in 

 part. If you place in a glass containing red or other colored fluid three or 



hunter's BIOL. — 8 



Diagram to show the areas in 

 the stem through which 

 raw food materials pass 

 up the stem and food 

 materials pass down 

 (After Stevens.) 



