OF THE MEDULLA OR PITH. 3 I 



plant, combining its various parts, but not performing 

 any remarkable oftice in the vegetable o^conomy. 



Linnaeus, on the contrary, thought it the seat of life 

 and source of vegetation ; that its vigour was the main 

 cause of the propulsion of the branches, and that the 

 seeds were more especially formed from it. This latter 

 hypothesis is not better founded than his idea, already 

 mentioned, of the pith adding new layers internally 

 to the wood. In fact, the pith is soon obliterated in 

 the trunks of many trees, which nevertheless keep in- 

 creasing, for a long series of years, by layers of wood 

 added every year from the bark, even after the heart 

 of the tree is become hollow from decay. 



Some considerations have led me to hold a medium 

 opinion between these two extremes. There is, in 

 certain respects, an analogy between the mtdulla of 

 plants and the nervous system of animals. It is no less 

 assiduously protected than the spinal marrow or prin- 

 cipal nerve. It is b^'anched off and diffused through 

 the plant, as nerves are through the animal. Hence it 

 is not absurd to presume that it may, in like manner, 

 give life and vigour to the wliole, though by no means, 

 any more than nerves, the organ or source of nourish- 

 ment. It is certainly most vigorous and abundant in 

 young and growing branches, and must be supposed 

 to be subservient, in some way or other, to their in- 

 crease. Mr. Lindsay of Jamaica, in a paper read long 

 ago to the Royal Society, but not published, thought 

 he demonstrated the medulla in the leaf-stalk of th( 



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