Movement of Materials in Stems 167 



almost wholly starch. The grains of wheat, oats, and rice contain 

 mostly starch, but also some protein. In sweet corn there are 

 both sugar ^ and starch ; in field corn the excess food is mostly 

 starch. In both sweet and field corn there are considerable 

 quantities of protein and oil. In the soy bean and peanut there 

 are large quantities of both protein and oil. In the seeds of the 

 coconut, flax, and cotton there is a large proportion of oil. 



Summary. The upward movement of water from roots to 

 leaves through the water-conducting tissue is due mainly to pull 

 of transpiration. Water moves from the water-conducting 

 tissues to other tissues of the stem by imbibition and osmosis. 



The movement of foods takes place mainly through the sieve 

 tubes and companion cells. Diffusion is known to be important 

 in this process, but it is inadequate to account for the rapid 

 transfer which occurs in many plants. 



Before insoluble foods move out of or into a cell, they must be 

 digested. This is done by enzymes, and there are probably as 

 many kinds of enzymes as there are classes of food substances. 

 The products of digestion are soluble substances. 



Both soluble and insoluble foods accumulate in stems and other 

 organs of plants. Insoluble foods are built up out of the soluble 

 foods that enter the cell by the same enzymes concerned in their 

 digestion. The reversal of the process is accounted for by certain 

 changes of conditions in the cell. 



PROBLEMS 



1. Why can a hollow tree continue to live for many years? 



2. Why do sprouts not develop from stumps of trees that were girdled a year before 



being cut down? 



1 Unless sweet com is cooked almost immediately after its removal from the 

 plant, it rapidly loses its sweetness. This is because the enzymes in the grains 

 constantly convert the sugar into starch. Peas and some other vegetables lose 

 their sweetness after being gathered, for the same reason. The enzymes work more 

 slowly at a low temperature, and the vegetables will lose their sweetness less rapidly 

 if kept in a refrigerator. 



