Page:Darwin - On the movements and habits of climbing plants.djvu/44

 here represented, the increase in diameter was not quite so great. In the section of the petiole in its ordinary state (A), we see a

semilunar hand of cellular tissue slightly different from that outside it, and including three closely approximate groups of dark vessels. Near the upper surface of the petiole, beneath two ridges, there are two other small circular groups of vessels. In the section of the petiole (B) which had during several weeks clasped a stick, the two upper ridges have become much less prominent, and the two groups of woody vessels beneath them much increased in diameter. The semilunar band is converted into a complete ring of very hard, white, woody tissue, with lines radiating from the centre. The three groups of vessels, which, though closely approximate, were before distinct, are now completely blended together. The upper part of the new ring of woody vessels, formed by the prolongation of the horns of the original semilunar band, is thinner than the lower part, and is slightly different in appearance from being less compact. This clasped petiole had actually become thicker than the stem close beneath; and this was chiefly due to the greater thickness of the ring of wood, which presented, both in transverse and longitudinal sections, a closely similar structure in the petiole and axis. The assumption by a petiole of this structure is a singular morphological fact; but it is a still more singular physiological fact that so great a change should have been induced by the mere act of clasping a support.

.—Fumaria officinalis.—It could not have been