SECT. It, COMPOSITE ORGANSi 21 1 



but as being also to the vegetable what the brain 

 and spinal marrow are to animals, the source and 

 seat of life. In these opinions there may be some- 

 thing of truth, but they have all the common fault 

 of ascribing to the pith either too little or too 

 much. 



Mr. Lindsay, of Jamaica, suggested a new opinion Lindsay, 

 on the subject, regarding it as being the seat of the 

 irritability of the leaves of the Mimosa, and Sir J. 

 E. Smith says he can see nothing to invalidate the 

 arguments on which this opinion is founded. Plenk Plenk,anfti 

 in his Physiologia Plantarum, and Mr. Knight in Knight, 

 one of his papers published in the Philosophical 

 Transactions, regard it as destined by nature to be a 

 reservoir of moisture to supply the leaves when ex- 

 hausted by excess of perspiration ; which opinion 

 Sir J. E. Smith combats, contending that the cause 

 assigned is wholly inadequate to the effect, as the 

 moisture of the pith would in many cases be insuf- 

 ficient to supply one hour's perspiration of a single 

 leaf, and as it is not found to be affected even when 

 the leaves are flaccid by drought. The objections 

 thus alleged are fatal to Mr. Knight's hypothesis, 

 which is regarded however as deriving considerable 

 support from the phenomena of the roots of some 

 of the Grasses, as from that of Phleum pratense, 

 which in moist situations has a fibrous root, while 

 in dry situations it has a bulbous root, the interior 

 of which is moist and spongy like the pith of the 

 young shoot. But this, instead of proving the pith 



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