THE STEM OF VASCULAR PLANTS. 191 



is regular in each species, the total length is suffi- 

 cient to give a pretty exact idea of the age of the 

 individual. 



Is the stem of Palms, as Linnssus has thought, and 

 as the stems of the Plantains would seem to demon- 

 strate, any thing more than the bmidle of the petioles 

 of the existing leaves sheathed by the hardened and 

 persistent ones of the old leaves ? It is in this point of 

 ^dew that the name of Frons, which signifies a leaf, or 

 Stipes, which means a support, has been given to it. 

 This hypothesis might be applicable, if one considered 

 it as a metaphor, but it can be but little followed as the 

 expression of the reality. 



The stem of Palms is, as I have explained, a cylin- 

 der, which grows indefinitely in height by its extremity, 

 and the thickness of which is determined in each spe- 

 cies by the time necessary to sohdify a layer after the 

 first period of its development. It sometimes happens 

 that the trunk presents here and there transverse con- 

 tractions, or swellings. These anomalies are owing to the 

 tree having had, at those periods, a slower or more vi- 

 gorous growth. There is in one of the greenhouses of 

 the Jardin-des-Plantes at Paris, a Cycas (a tree analo- 

 gous to the Palms in the structure of its stem) which 

 has, in the middle of its length, a very distinct contrac- 

 tion, which corresponds to the period when it was being 

 brought from the Isle of France to Paris. During this 

 transportation it received but Kttle nourishment, and 

 the solidifying of the external fibres took place before 

 they had attained their full size. Similar contractions 

 can never take place in Exogens ; and, on the other 

 hand, Palms, and other Endogens, can never present 

 lateral exostoses, since all their fibres are longitudinal ; 

 and the external ones, already hardened, form a kind of 

 sheath around the younger ones. 



