122 



Science of Plant Life 



Fig. 70. Cross section of a portion of 

 rootstock of calamus, photographed 

 through a microscope. The circular 

 areas are the cells which are filled with 

 starch. 



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. Be- 

 cause the cells of the stem and 

 root are supplied with food 

 manufactured in the leaves, it 

 is often said that the move- 

 ment of foods is downward in 

 a plant. In reality, the direc- 

 tion 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 is being accumulated. For example, in mid- 

 summer when a tree is in full leaf and the season's growth 

 has practically been completed, food moves out of the smaller 

 branches into the larger branches and the trunk. In the 

 spring, when leaves and blossoms are developing, food is 

 being used in the twigs, and the direction of the movement 

 of food materials is reversed. 



The mechanical tissue is made up of cylindrical or spindle- 

 shaped cells with very heavy walls. Indeed, the walls at 

 maturity may be so thick as to render the cells almost 

 soHd. Ordinary cellulose is not very hard, but the walls 

 of the mechanical tissue are hardened and thickened 

 by a deposit of a substance called lignin. The differ- 

 ence between hard and soft woods is for the most part 

 due to the thickening of the walls of the mechanical cells; 



