The branches on slow-growing limbs of cottonwood and large-toothed aspen are much enlarged at the nodes, and at these places are brittle, often separating from the tree and breaking up into pieces. Under a small cottonwood were picked up a bushel or more of such limbs, all yet alive. These trees are common on low land, and, like snap-willows, the severed twigs may find a chance to grow on moist soil.[2] [Footnote 2: The brittle branches of salix were noticed by the author in _Bull. Torr. Bot. Club_, Vol. IX (1883), p. 89.] [Illustration: FIG. 10.--Portion of a branch of the cottonwood as it fell from the tree.] In a greenhouse a potted plant of _Selaginella emiliana_(?) was placed on the bench near the aisle, where it was often brushed by people in passing. Small branches, not being firmly attached, were frequently broken from the main plant and fell upon the moist sand, where they rooted in abundance. CHAPTER IV. WATER TRANSPORTATION OF PLANTS. 11. Some green buds and leaves float on water.--Loosely floating on slow streams of the northern states, in water not the purest, may often be found the common bladderwort, _Utricularia vulgaris_, producing in summer a few yellow flowers on each stem, rising from six to twelve inches above the water. The lax, leafy branches in the water are from six inches to a foot long. The leaves, or thread-like branches, are about half an inch long, more or less, and several times divided. [Illustration: FIG. 11.--A free branch and two buds of bladderwort.]