TWINING PLANTS. Plants whose axes are coiled about slender supports, e.g. common morning-glory and hop. In most cases the support must be slender (less than 15 centimeters in diam eter). though some tropical plants coil about thick tree trunks, and others climb in a some what similar fashion by partial twining. (See LIANAS.) The direction of twining varies with different plants, being either clockwise or coun ter-clockwise. While the direction of twining is usually constant in the same species. some twin ers coil in either direction. The direction is not constant, however, among members of the same family, nor even among different species of the same genus.
The cause of twining has long been a problem of great difficulty. The most satisfactory expla nation seems to be the following: Twining stems are endowed with a sensitiveness to the action of gravity, which may be distinguished as lateral geotropism (q.v.). The stem at first grows erect (on account of negative geotropism), but through unequal growth, causing notation (q.v.), soon in clines to one side. At this period of its develop ment, the flanks (i.e. the sides of the stem now on the right and left as distinguished from the upper and lower) are sensitive to their changed relation to gravity (on account of the new horizontal posi tion), which thus becomes a stimulus. The re sponse of the stem to this stimulus is an accel erated growth on one think, the right or left, as the case may he. The acceleration of growth there swings the horizontal tip toward a new point of the compass, and at the some time, from mechanical necessity, partially rotates the stein on its longitudinal axis, so that a new side is brought into the flank position and therefore under stimulation. It then responds hkewise with accelerated growth; this swings the tip farther around, and again brings a new region under stimulation. By this continued action each side of the stem is successively stimulated, and the tip is therefore swung in a circle. At
the same time it is elongating. If, in the course of its twining, the nearly horizontal tip strikes a support, only the part beyond the support con tinues to swing, and therefore begins to wrap around the support, forming coils which are at first low and loose. At a later period negative geotropism again asserts itself in this part of the stem, which grows so that it straightens, if possible, the actual effect being to steepen the coils and make them hug the support more close ly. The slenderer the support, as a rule, the steeper are the completed coils. This explana tion receives its strongest support from the fact that when the action of gravity is equalized, as it can be by rotating a seedling twiner on a clin ostat (q.v.). no coiling takes place. There are various phenomena of twining, experimentally in duced, of which no full explanation can yet be given. There is, however, no mere mechanical stoppage of the swinging tip which permits coil ing, because it has no momentum, and because many twiners will coil about a loose cord. It is further probable that twiners are susceptible to the continued contact and pressure of the sup port, and that this plays some part in the method of climbing which they have adopted.
Twining plants are characterized by unusual ly long growing regions, and by the retarded de velopment of foliage leaves. Thus, while in erect stems from 10 to 20 centimeters (4 to S inches) of the apex are still capable of elongation, in twining plants the elongating region is 30 to 50 centimeters (12-20 inches) long, or in some cases as SO centimeters; (32 inches). The slow development of the foliage leaves seems both to be correlated with this long maintenance of the power of growth, and to be necessary for twin ing, because the presence of fully developed foli age would interfere seriously with the free move ments of the tip.