122 



Science of Plant Life 



account they are called sieve 

 tubes. Through the openmgs 

 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 suppHed with food 

 manufactured in the leaves, it 

 ,. , ^. , is often said that the move- 



FiG. 70. Cross section of a portion of 



rootstock of calamus, photographed meut of foods is doWnWard in 

 through a microscope. The circular ^ ^^^^ j^ reality, the dlrCC- 

 areas are the cells which are filled with 



starch. tiou of the food currcut 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 cyhndrical or spindle- 

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

 maturity may be so thick as to render the cells almost 

 solid. 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 ; 



