C -6 ] 



form, and similar effects are usually produced by simi- 

 lar organs. 



The pith occupies the centre of the wood ; its 

 texture is membranous ; it is composed of cells, which 

 are circular towards the extremity, and hexagonal in 

 the centre of the substance. In the first infancy of 

 the vegetable, the pith occupies but a small space. It 

 gradually dilates, and in annual shoots and young 

 trees offers a considerable diameter. In the more 

 advanced age of the tree, acted on by the heart-wood, 

 pressed by the new layers of the alburnum, it begins 

 to diminish, and in very old forest trees disappears 

 altogether. 



Many different opinions have prevailed with re- 

 gard to the use of the pith. Dr. Hales supposed, that 

 it was the great cause of the expansion and develope- 

 ment of the other parts of the plant j that being the 

 most interior, it was likewise the most acted upon of 

 all the organs, and that from its reaction the pheno- 

 mena of their developement and growth resulted. 



Linnaeus, whose lively imagination was continu- 

 ally employed in endeavours to discover analogies be- 

 tween the animal and vegetable systems, conceived 

 " that the pith performed for the plant the same func- 

 tions as the brain and nerves in animated beings.'* He 

 considered it as the organ of irritability, and the seat 

 of life. 



The latest discoveries have proved, that these two 

 opinions are equally erroneous. Mr. Knight has re- 

 moved the pith in several young trees, and they con- 

 tinued to live and to increase. 



. 



