THE STEM OF VASCULAR PLANTS. 147 



the Chestnut, the Catalpa, &c. It appears that Hill 

 was the first who observed this organ ; he has named it 

 Corona, and regards it as the principal agent of vege- 

 tation. In several trees, and especially in those with 

 alternate leaves, the pith forms a continuous canal from 

 one end of the tree to the other, but plainly contracted 

 at each new shoot. In others, on the contrary, such as 

 the Chestnut, the Ash, the Vine, which have all oppo- 

 site leaves, the pith is interrupted, at each node 'or 

 annual shoot, by a kind of woody partition. The same 

 thing takes place, in a still more evident manner, in 

 articulated stems ; for example, in that of Cacalia arti- 

 culata. 



The mass of cellular tissue of the pith varies much in 

 different species : herbs and shrubs have generally more 

 than trees. The Ferula has it the largest that I have 

 ever seen, in proportion to the diameter of the stem. 

 Among trees, those with very hard wood seem in 

 general to have less than others : the Ebony (Dios- 

 pyros Ehenum) and Lignum Vita; (Guiacum officinale) 

 have very little ; the Pear and Oak a little more ; the 

 Elder, the White Thorn, the Fig, the Sumach, and the 

 Chestnut, have still more. 



The size of the cellules of the pith is also very vari- 

 able, if different species are compared together. The 

 Elder and Thistle have both a large quantity of pith ; 

 but in the former it is composed of a great number of 

 very small cellules : in the latter, of a less number of 

 much larger ones. 



The Medullary Canal in young shoots, instead of 

 being cjlindrical, frequently presents regularly placed 

 angles. These are connected with the disposition of the 

 leaves upon the branch. This interesting observation of 

 Palisot de Beauvois and Du Petit-Thouars, has not 

 been applied to a sufficiently large number of species to 



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