3 C 2 OF THE MEDULLA Oil PITH. 



Mimosa pudica, or Sensitive Plant, to be the seat of 

 irritability, nor can I see any tiling to invalidate this 

 opinion. 



Mr. Knight, in the Philosophical Transactions for 

 1 80 1 , /;. 348, supposes the medulla may be a reser- 

 voir of moisture, to supply the leaves whenever an ex- 

 cess of perspiration renders such assistance necessary, 

 and he has actually traced a direct communication by 

 vessels between it and the leaf. " Plants," says that 

 ingenious writer, " seem to require some such reser- 

 " voir ; for their young leaves are excessively tender, 

 " and they perspire much, and cannot, like animals, 

 " fly to the shade and the brook." 



This idea of Mr. Knight's may derive considerable 

 support from the consideration of bulbous-rooted 

 grasses. The common Cats-tail, Phleum pratense, 

 EngL Bot. t. 1076', when growing in pastures that are 

 uniformly moist, has a fibrous root ; but in dry situ- 

 ations, or such as are only occasionally wet, it ac- 

 quires a bulbous one, whose inner substance is moist 

 and fleshy, like the pith of young branches of trees. 

 This is evidently a provision of Nature to guard the 

 plant against too sudden a privation of moisture from 

 the soil. 



But, on the other hand, all the moisture in the 

 medulla of a whole branch is, in some cases, too little 

 to supply one hour's perspiration of a single leaf. 

 Neither can I find that the moisture of the medulla 

 varies, let the leaves be ever so flaccid. I cannot but 



