38 PLANT LIFE ON THE FARM. 



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 complicated, is in the first instance quite simple, 

 being merely an aggregation of cells. Another stem, of very 

 different general appearance it may be, yet began in pre- 

 cisely the same way. It is only necessary here to allude in 

 passing to the variations in internal structure, according to 

 circumstances, 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 inquire 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 neigh- 

 bour 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 favourable circum- 

 stances, and hinder its neighbour to the least possible 

 degree. One leaf would not be of much use, but the 

 aggregation of many produces a timber tree. One leaf's 

 work would probably not suffice to build up a grain of 

 wheat, the aggregation of them serves to form a sheaf of 

 plump ears. The stem, in fact, is the agency by which 

 the work of individual leaves is combined and concentrated 



