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It had long been known, that in the "sieve" tissues of higher plants there was such continuity through the "sieve plates," which imperfectly separated the contiguous cells. This may be readily seen by making longitudinal sections of a fibro-vascular bundle of a pumpkin stem, staining with iodine, and contracting the protoplasm by alcohol.
A kind of cell, not strictly woody, is where many cells form long vessels by the breaking away of the connecting walls. These are ducts. These two kinds of cells are generally associated together in woody bundles, called therefore fibro-vascular bundles. We have already spoken of them as making the dots on the leaf-scars, and forming the strengthening fabric of the leaves.
Let the pupils compare the branches which they have described, with a stalk of Asparagus, Rattan, or Lily. A cross section of one of these shows dots among the soft tissue. These are ends of the fibro-vascular bundles, which in these plants are scattered through the cellular tissue instead of being brought together in a cylinder outside of the pith. In a vertical section they appear as lines.
"That water finds its way by preference through the fibro-vascular bundles even in the more delicate parts, is shown by placing the cut peduncle of a white tulip, or other large white flower, in a harmless dye, and then again cutting off its end in order to bring a fresh surface in contact with the solution, when after a short time the dye will mount through the flower-stalk and tinge the parts of the perianth according to the course of the bundles."
The scales of the flower-buds are somewhat gummy, but not nearly so much so as those of the leaf-buds. Within is the catkin. The leaf-scars are somewhat three-lobed on the young parts, with three dots, indicating the fibro-vascular bundles, which ran up into the leaf. The scars are swollen, making the young branches exceedingly rough. In the older parts the scars become less noticeable.
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