118 PLANT FOODS AND PLANT GROWTH 



nets are called humus, and we have long recognized that humus 

 is essential for plant growth. The second shows the importance 

 of the use of leguminous plants to increase the nitrogen content 

 of the soil and explains why clovers, alfalfa, cowpeas, soybeans, 

 vetch and similar plants are of such great importance as soil 

 improvers. 



Circulation. Knowing that plants take in water from the 

 soil, that it rises through the plants and is given off through the 

 foliage, naturally indicates that the plant possesses a circulatory 

 system similar to that of animals. Although the circulation, is 

 through definite parts of the plant tissues, it is not through a 

 definite set of tubes and does not have a propelling organ or 

 pump such as the heart of the animal. The water rises in the 

 wood (or xylem) part of the stem and carries the mineral 

 substances of the soil in solution. But this does not complete 

 the problem of circulation. Very little of the manufactured 

 plant food is stored in the parts in which it is formed but is 

 transferred to fruits, seeds, tubers, bulbs, roots and other parts 

 for storage. This transfer is primarily through the outer (or 

 phloem) part of the stem. We do not understand just how 

 this transfer is accomplished, but much of the food must undergo 

 modifications, must become soluble, before it can be moved. 

 The carbohydrate is probably moved while in the form of sugar 

 (glucose), and the fats and true soils as glycerine and fatty 

 acids, and the proteids in a form known as amides. 



EXERCISES SHOWING PROCESSES IX PLAXT GROWTH 



1. Osmosis Through a Bladder. Tie a bit of bladder or sausage 

 casing or parchment over the large end of a thistle tube; partly fill the 

 tube with molasses and immerse the large end in water until the two 

 liquids are on a level. Make observations at intervals of one hour and 

 explain the phenomena. 



2. Exosmosis and Endosmosis- Peel some fleshy root such as car- 

 rot, turnip or beet and cut in slices about % x 1% inches and about l / s 

 inch in thickness. Put a few of them in distilled water and a few in a 

 salt or sugar solution. Examine after a few hours and explain. Now 



