42 THE STEM. 



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, tlie first vital functions ; but after 

 the young shoot has become organized so as to derive nour- 

 ishment from other sources, the now useless pith becomes 

 dry ; being exhausted of its fluids and often torn and vari- 

 ously divided by tlie growth of the stem. It not unfrenuontly 

 entirely decays, thus showing that it is necessary only in the 

 early stages of the plant. 



There are frequently to be found in the bark of several 

 trees, particularly of the Beach, small conical bodies compo- 

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

 They are generally, in their early stage at least, not con- 

 nected with the subjacent wood. Dutrochet believes nodules 

 to be adventitious buds, wliich generally do not acquire force 

 sufficient for their development into branches ; but in some 

 cases they do produce branches which are of a weakly 

 character. 



49. ErcDOGENOus Stems, are such as are formed by bun- 

 dles of vascular tissue and woody fibre imbedded irregularly 

 in cellular substances, and they increase by the deposition of 

 vegetable substance internally. They differ from those above 

 described in not having bark, pith, or medullary rays. The 

 above definition of endogenous stems will apply in general 

 to this division of vegetal)les, but there are three varieties of 

 them, which will require our separate notice. First, such as 

 are uniform in their structure throughout their length, and 

 grow to be trees. We may take as a type of this structure^ 

 the Yucca gloriosa, or the Chammrops palmetto. If we make 

 a transverse section of one of these stems, we find a cylinder of 

 spongy substance with numerous strong, large bundles made 

 up of woody fibre and vascular tissue imbedded in it. This 

 cylinder is surrounded by three distinct zones. The outer 

 one consists of the basis of the leaves which clothed the stem. 

 Within this is a zone of cellular substance, which may be 

 seen in fig. 16, a. The next zone (6,) is composed of mate- 

 rials like the liber of the bark, and thev 

 have tlie same arrangejnent. This 

 zone is considered by Botanists as of 

 the same construction as the internal 

 cylinder, but we believe this is not cor- 

 rect, since it may be separated into 

 layers whereas tlie fibres of the central 

 part, traverse the pulpy substance in 

 every direction, and nothing approach- 

 ing a regular arrangement can be discovered. We believe 



