Page:An introduction to physiological and systematical botany (1st edition).djvu/71

Rh 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. "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 grasses. The Common Catstail, Phleum pratense, Engl. Bot. t. 1076, when growing in pastures that are uniformly moist, has a fibrous root, but in dry situations, or such as are only occasionally wet, it acquires 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