THE CULTURE OF FARM CROPS. 



CHAPTER XXXVI. 



THE FUNCTIONS OF THE STEMS. 



The stem performs two offices as the roots also do; to 

 sustain the leaves and fruit, and to convey from the roots 

 to the leaves the nutriment which the former have gathered 

 from the soil ; as well as to return to the roots whatever sap 

 or nutritious matter the roots require for their growth; to- 

 gether with any excess of it which has not been used by 

 the plant. It may be questioned if there is ever any ex- 

 cess of such matter taken into the plant, and if the roots 

 do not absorb precisely so much and no more food than the 

 plant requires. But we have seen that the roots do dis- 

 charge from the plant some matter which is in excess of 

 that required, and it is probable that a constant circulation 

 always goes on through the plant, from the roots through 

 the stem to the leaves, and back through the stem to the 

 roots. 



Plants are divided by botanists into two great classes, 

 Endogenous, or those w y hose stem increases by additions of 

 cellular tissue within it; and Exogenous, or those whose 

 stems grow by additions to the outside. The first class is 

 represented by the palm tree, and all the great grass fam- 

 ily of plants, which include corn, wheat, sorghum, and all 

 those plants which have hollow and jointed stems, or a pithy 

 center surrounded by a hard outer casing; but do not have 

 bark; the other class is represented by the majority of 

 plants, which like trees, increase by layers of tissue on the 

 outside of the stem between the wood and the bark. The 

 plants of greater interest to the farmer belong chiefly to 

 the first class; but in both, the stem exerts similar functions 

 and has similar relations to the growth of the plants. 



The stem is a prolongation of the upper portion of the 

 root, and consists of a mass of longitudinal, hollow, tubular, 

 vessels, through which the sap circulates from the root to 



