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. 
Uses 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 te 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, 
