PLANT NUTRITION. 41 



great extent of variation there is in its outward charac- 

 teristics. The inward conformation of the stem varies 

 according to the nature of the plant its age and the 

 purpose it fulfils. The structure of the stem of a timber 

 tree and that of a potato tuber both, as we have seen, 

 forms of stems are naturally different. In the one case 

 thin- walled cells filled with starch predominate, in the 

 other wood-cells and fibres filled with woody matter are 

 most abundant. Still, great as are the differences in the 

 manner in which the structural elements are arranged in 

 different cases, those elements are precisely the same as 

 those mentioned as existing in the root and in the leaf ; 

 and the structure of a stem, however ultimately compli- 

 cated, is in the first instance quite simple, being merely 

 an aggregation of cells. Another stem, of very different 

 general appearance it may be, began in precisely the 

 same way. It is only necessary here to allude in passing 

 to the variations in internal structure, according to cir- 

 cumstances, as they must necessarily be referred to again 

 when dealing with the office of the stem and its mode of 

 growth. 



Ises of the Stem the Sap. Having gained a general 

 notion of the nature and construction of the stem, it is 

 necessary to enquire as to its office. What does it do for 

 the plant ? The answer to this may in a measure be 

 gleaned from what has been said as to the office of the 

 leaves. The necessity for their exposure to sunlight has 

 been shown, and to ensure this exposure, and to provide 

 that one leaf shall overshadow and interfere with its 

 neighbor as little as possible, the stem lengthens, and 

 the leaves are thrown off, now on this side, now on that, 

 so that each shall do its own work under the most favor- 

 able circumstances, and hinder its neighbor to the least 

 possible degree. One leaf would not be of much use, 

 but the aggregation of many produces a timber tree. 



