48 OF THE MEDULLA OR PITH. 



has actually traced a direct communication by vessels 

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

 writer, " seem to require some such reservoir ; 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 grass- 

 es. The Common Catstail, Phleum pratense^ Engl. 

 Bot. t. 10'76, when growing in pastures that arc uni- 

 formly moist, has a fibrous root, but in dry situations, 

 or such as are only occasionally wet, it acquires a bul- 

 bous one, whose inner substance is moist and fleshy, 

 like the pith of young branches of trees. This is evi- 

 dently 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 ?72f- 

 dulla 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 incline 

 therefore to the opinion that the medulla is rather a re- 

 servoir of vital energy, even in these bulbous grasses. 



M-. Knight has shown that the part iii question may 

 be removed without any great injury to a branch, or at 

 least without immediate injury, but I have had no op- 

 portunity of making any experiments on this particular 

 subject. 



