In the common _Yucca_, the more tropical species have erect stems; but in the form known in gardens as Adam’s needle and thread—_Yucca filamentosa_—the erect stem is sent down under the surface of the ground, and is then a rhizome, instead of a caudex, or stem.
Modification in connection with behavior is further illustrated by the grapevine and Virginia creeper. The whole leading shoot is here pushed aside by the development of a bud at the base of the leaf, that takes the place of a leading shoot. The original leader then becomes a tendril, and serves in the economy of the plant by clinging to trees or rocks, or in coiling around other plants in support. Great progress has been made in this department of botany within recent years. Darwin has shown that the tendrils of some plants continue in motion for some time in order to find something to cling to. The grapevine especially spends a long time in this labor if there is difficulty in reaching a host. The plant preserves vital power all this time, but no sooner is support found, than nutrition is cut off, and the tendril dies, though, hard and wiry, it serves its parent plant as a support better dead than alive. The amount of nutrition spent in sustaining motion is found to be enormous. A vine that can find ready means of support grows with a much more healthy vigor than one that has difficulty in finding it. Many plants present illustrations.
Much advance has been made in the knowledge of the motions of plants as regards their various forms. Growth in plants is not continuous; but is a series of rests and advances. In other words it is rhythmic. The nodes, or knots, in the stems of grasses are resting-places. When a rest occurs, energy may be exerted in a different direction, and a change of form result. This is well illustrated by the common Dogwood of northern woods, _Cornus florida_ on the eastern, and _Cornus Nuttallii_ on the western slope of the American continent. On the approach of winter the leaf is reduced to a bud scale, and then rests. When spring returns these scales resume growth and appear as white bracts. In the annexed illustration the scales that served for winter protection to the buds are seen at the apex of the bracts. In other species of Dogwood the bud scales do not resume growth. Energy is spent in another direction. In this manner we have an insight as to the cause of variation, which was not perceived even so recently as Darwin’s time. We now say that variation results from varying degrees of rhythmic growth—force; and that this again is governed by varying powers of assimilation.