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This medullated sheath is interrupted at the nodes of Ranvier, and in each internode is a nucleus lying between the myelin and the neurolemma. The axis cylinder is the essential conducting structure of the nerve, while the neurolemma and the myelin act as insulating agents.
The larger leaflets are sometimes entire, but more generally notched, cut, or even divided, particularly at the base. =The flowers= are pendant and borne in more or less branched clusters, located on the stem on the opposite side and usually a little below the leaves; the first cluster on the sixth to twelfth internode from the ground, with one on each second to sixth succeeding one.
Some time before the last revolution was half performed, the lower part of the internode ceased to move. A few more remarks will complete all that need be said about this internode. It moved during five days; but the more rapid movements, after the performance of the third revolution, lasted during three days and twenty hours.
Each branch is curved a little downwards, and is slightly hooked at the extremity. A young upper internode revolved, judging from three revolutions, at an average rate of 1 hr. 38 m.; it swept ellipses with the longer axes directed at right angles to one another; but the plant, apparently, cannot twine. The petioles and the tendrils are both in constant movement.
These at once show the spiral line and it is easy to follow this line from the base up to the apex. In the most marked cases it continues without interruption, not rarely however, ending in a whorl of three leaves and a subsequent straight internode, of which there may even be two or three. The spiral exhibits the basal parts of the leaves, with the axillary lateral branches.
I then moved the stick to a greater distance, so that it was struck by a point 2.5 inches from the extremity of the penultimate internode; and it was then neatly encircled by this part of the penultimate and by the ultimate internode.
There was only one irregular movement, which consisted in the stem rapidly making, after an unusually slow revolution, only the segment of a circle. After the seventeenth revolution the internode had grown from 1.75 to 6 inches in length, and carried an internode 1.875 inch long, which was just perceptibly moving; and this carried a very minute ultimate internode.
When the shoot follows the sun in its revolving course, it winds round the support from right to left, the support being supposed to stand in front of the beholder; when the shoot revolves in an opposite direction, the line of winding is reversed. As each internode loses from age its power of revolving, it likewise loses its power of spirally twining.
The stem, as the scholars have already learned, is the axis of the plant. The leaves are produced at certain definite points called nodes, and the portions of stem between these points are internodes. The internode, node, and leaf make a single plant-part, and the plant is made up of a succession of such parts. The stem, as well as the root and leaves, may bear plant-hairs.
A familiar illustration of this tension will be found in the Dandelion curls of our childhood. By Geo. L. Goodale. Ivison & Co., New York, 1885. "From a long and thrifty young internode of grapevine cut a piece that shall measure exactly one hundred units, for instance, millimeters.
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