OF THE MEDULLA OR PITH, 41 



the leaves whenever an excess of perspiration 

 renders such assistance necessary, and he 

 has actually traced a direct communication 

 by vessels between it and the leaf. a Plants," 

 says that ingenious writer, " seem to require 

 some such reservoir; for their young leai 3 

 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 con- 

 siderable 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 situa- 

 tions, or such as are only occasionally wet y 

 it acquires a bulbous one, whose inner sub- 

 stance 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 perspi- 

 ration of a single leaf. Neither can I find 



