32 THE STEM. 



but the line of demarkation is often perfect ; the black external 

 layer of the perfect wood being surrounded by a perfectly white 

 layer of alburnum, thus showing that the transition is performed 

 at once, and not gradually, as is generally supposed. 



58t The pith is the central portion of the stem, commencing at 

 its base and extending through it and through every branch, ter- 

 minating in the buds. It is composed of loose cellular substance, 

 varying considerably in size, form, and appearance in different 

 plants. In some plants it forms a large portion of the stem, as in 

 the Elder ; in others but a small part ; and in trees, becomes com- 

 pressed into a mere line, as in the Oak. It never increases in 

 quantity in the same part of the stem. Its only function seems 

 to be that of nourishing the young buds. During its early 

 stages it is filled with fluid, and performs, undoubtedly, the first 

 vital functions : but after the young shoot has become organized 

 so as to derive nourishment from other sources, the now useless 

 pith becomes dry ; being exhausted of its fluids and often torn 

 and variously divided by the growth of the stem. It not un- 

 frequently entirely decays, thus showing that it js necessary only 

 in the early stages of the plant. 



59. There are frequently to be found in the bark of several 

 trees, particularly of the Beach, small conical bodies composed 

 of wood, pith, and medullary rays, which are called nodules. 

 They are generally, in their early stage at least, not connected 

 with the subjacent wood. Dutrochet believes nodules to be ad- 

 ventitious buds, which generally do not acquire force suflScient 

 for their development into branches ; but in some cases they do 

 produce branches, which are of a weakly character. 



60. Monocotyledonous stems are characterized by having the 

 difterent portions less distinctly marked than they are in the pre- 

 ceding class, having parallel-veined leaves, the number of floral 

 organs three or six, and not having any medullary rays or dis- 

 tinct bark or pith. 



The monocotyledonous stem increases, as all other vegetables 

 do, by the deposition of the new matter exterior to the old. 

 The peculiarity of this growth depends mainly upon the fact, 

 that the new leaves are formed on nearly the same plane with 

 older ones. Shorten the internodes of an oak, and let the leaves 

 become formed and produce new matter before they rise above 

 the older ones, and you would essentially convert it into a mo- 

 nocotyledonous growth. You would destroy in a great measure 



58. What is the pith? How tho quantity in different plants? Does it 

 ever increase in quantity? How in its earliest stage? — 59. What are nod- 

 ules ? — 60. How are monocotyledonous stems characterized ? How do 

 their stems increase ? On what does this pecul arity depend ? 



