134 General Botany 



and cylindrical living cells. All together these tissues form the 

 passageway for the movement of water and mineral salts to all 

 parts of the plant. The general direction of the water move- 

 ment in this tissue is upward, because the lifting of the water is 

 brought about principally by transpiration from the leaves. 



The simplest land plants are very small and grow flat on the 

 soil in wet places. They are constantly in contact with the moist 

 soil, and their cells can be supplied almost directly with water 

 and mineral salts. In such plants a conductive system is not 

 necessary ; but if the leaves of a plant are to be raised into the 

 air, water lost by transpiration must not only be supplied to them 

 continuously, but at times it must be supplied in great quantity. 

 Because of this fact, a plant that raises its leaves even a few inches 

 above the soil must possess conductive tissues, and when large 

 numbers of leaves are raised 200 or 300 feet into the air, a very 

 extensive water-conducting system is necessary. 



The food-conducting tissue differs from the water-conducting 

 tissue in being composed of smaller, thin-walled cells, all of which 

 retain their living protoplasm. The largest of these cells are set 

 end to end, and the end-walls have holes in them like the top of a 

 salt shaker. These rows of cells, therefore, form tubes with 

 sieve-like cross-walls in them, and on this account they are called 

 sieve tubes. Through the openings in the sieve plate the proto- 

 plasm is continuous from cell to cell, and through these tubes 

 the foods pass from one part of the plant to another. Surround- 

 ing the sieve tubes are smaller living cells called companion cells. 

 Because the cells of the stem and root are supplied with food 

 manufactured in the leaves, it is often said that the movement of 

 foods is downward in a plant. In reality, the direction of the 

 food current is not so fixed as is that of the water current. Food 

 moves toward any part of the plant where it is being used or 

 being accumulated. 



The roots and stems require a continuous supply of food for 

 nourishing old cells and for building new ones. Since the foods 



