THE STEM OF VASCULAR PLANTS. 149 



in herbaceous stems, in which the medullary canal is 

 hollowed out into a longitudinal tube, either in the first 

 year, or sometimes a little afterwards, when the first 

 woody layer begins to dilate. 



But what becomes of this pith after the first year ? 

 This question is, in reality, more curious than useful ; 

 for the pith, being dried up and inert, does not appear 

 to have any function. Grew was the first who advanced 

 that it was smaller in a branch of two years' growth than 

 in that of one ; that it was still more diminished in one 

 of three years ; and thus it continued to decrease as it 

 advanced in age : whence he appears to infer that it 

 vanishes after some time. Duhamel positively asserted 

 this disappearance of the pith in old trunks ; " by de- 

 grees," he says, " the medullary canal diminishes in 

 diameter; and in large trees (those same which, in 

 their young state, have most pith), neither the canal or 

 medullary substance are any longer to be seen." (Phys. 

 Arb. i. p. 37.) 



Mustel also admits this disappearance of the dried 

 pith, and the formation of new woody layers in the inte- 

 rior of the medullary canal {Traite Vigtt. i. p. 62). 



Mirbel also says, in his Histoire des Plantes, Vol. I. 

 p. 194, that an interior liber (of which he admits the 

 existence) is developed, and that the pith completely dis- 

 appears. Almost all modern authors agree with this 

 opinion. Senebier appears to admit as a certain fact 

 this disappearance of the pith, since he searches for 

 means to explain it ; but he appears to beheve that this 

 phenomenon is not general in all trees. Varennes de 

 Fenille has been the first to remove the doubt from the 

 assertion of Duhamel, by saying that he possesses two 

 specimens which prove the contrary. But in these later 

 times, Mr. Knight (Philosophical Transactions, 1801) 

 and Du Petit-Thouars appear to me to have brought to 



