OF THE MEDULLA OR VITH. ^ 



cause of the propulsion of the branches, and that the 

 seeds were more especially formed from it. This hit- 

 ter hypothesis is not better founded than his idea, alre.idy 

 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 increas- 

 ing, 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 cer- 

 tain respects, an analogy between the medulla of plants 

 and the nervous system of animals. It is no less assid- 

 uously protected than the spinal marrow or principal 

 nerve. It is branched 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 whole, though by no means, any 

 more than nerves, the organ or source of nourishment. 

 It is certainly most vigorous and abundant in young and 

 growing branches, and must be supposed to be subser- 

 vient, in some way or other, to their increase. Mr. 

 Lindsay of Jamaica, in a p^iper read long ago to the 

 Royal Society, but not published, thought he demon- 

 strated the medulla in the leaf-stalk of the Mimosa pu- 

 dica, or Sensitive Plant, to be the seat of irritability, nor 

 can I sec any thing to invalidate this opinion. 



Mr. Knight, in the Philosophical Transactions for 

 1801, j&. 348, supposes the medulla may be a reservoir 

 of moisture, to supply the leaves whenever an excess of 

 perspiration renders such assisunce necessary, and he 



