THE S1?EM AND iJItANCHES. 179 



of protoplasm and chlorophyll. The third is, to 

 carry away and distribute the various matured 

 products of plant life, such as starches, sugars, 

 oils, and protoplasm, from the places in which 

 they are produced (such as the leaves) to the 

 places where they are needed for building up 

 the various parts of the compound organism 

 (such as the flowers and fruit or the growing 

 shoots), as well as to the places where such 

 materials are to be stored up for safety or for 

 future use (as, for example, the tubers and roots, 

 or the buds, bulbs, and other dormant organs). 

 Each of these three essential functions we must 

 now proceed to consider separately. 



In order to raise the leaves and branches 

 visibly above the ground into the air above it, 

 the stem is made much stronger and stouter 

 than the ordinary leaf -tissue. If the plant does 

 not rise very high above the ground, indeed, as 

 in the case of small herbs, and especially of 

 annuals, its stem need not be very hard or stiff, 

 and is often in point of fact quite green and 

 succulent. But just in proportion as plants 

 grow tall and spreading, carry masses of foliage, 

 and are exposed to heavy winds, do they need 

 to form a stout and woody stem, which shall 

 support the constant weight of the leaves, or 

 even bear up under the load of snow which may 

 cover the boughs in wintry weather. Thus, a 

 tapering tree like the Scotch fir requires a com- 

 paratively smaller stem than an oak, because its 

 branches do not spread far and wide, wliile its 

 single leaves are thin and needle-like;, whereas 



