THE STEM OF VASCULAR PLANTS. 143 



are not in an equal position ; those on the upper ought 

 not to grow as much as those on the lower side, because 

 they cannot go above their level, and at a certain depth 

 they do not feel the beneficial influence of the atmo- 

 sphere; those of the lower surface, on the contrary, 

 ought to grow with the greatest facility ; consequently, 

 the branches of the lower side will elongate more than 

 those of the upper : but tlie longest branches are those 

 which, by their weight and tendency towards the light, 

 are obliged to spread out more ; the lovt'er branches then 

 will be more spreading than the upper, whence results 

 this rude parallelism of the branches with the ground : 

 and in fact it is only apparent in trees with spreading 

 branches, and we always remark that the upper side of 

 the cyme is smaller than the other. 



Section II. 



Of the Stems of Exorjens or Dicotyledons. 



The stem of Exogens presents an organization more 

 complicated than all others ; and if I commence by ex- 

 amining this class, I do so because it is much better 

 known than the two others ; because the multiplicity 

 even of its organs causes each to perform a function 

 more easy to describe ; and because, containing more- 

 over all the trees of our climate, it naturally presents 

 itself to our notice. 



We meet with, at first sight, two very distinct parts 

 in the stem of Exogens — viz. the Woody Body, or 

 Central System (corps Ugneux, ou sysieme central), 

 which is placed in the centre, and forms the principal 

 part of the trunk ; and the Cortical Body or System 

 {corps ou sysieme cortical), or the Bark, which surrounds 



