Fig. 90.  Cotyledon umbilicus:  circumnutation and downward movement of another stolon, traced on vertical glass, from 9.11 A.M.  Aug. 25th to 11 A.M. 27th.  Apex close to glass, so that figure but little magnified, and here reduced to two-thirds of original size.

served from 6.40 A.M. to 5.20 P.M., and within this time it moved twice up and twice down.  On the morning of the 27th the apex stood as high as it did at 11.30 A.M. on the 25th.  Nor did it sink down during the 28th, but continued to circumnutate about the same place.

Another stolon, which resembled the last in almost every [page 222] respect, was observed during the same two days, but only two inches of the terminal portion was allowed to project freely and horizontally.  On the 25th it continued from 9.10 A.M. to 1.30 P.M. to bend straight downwards, apparently owing to its weight (Fig. 90); but after this hour until 10.35 P.M. it zigzagged.  This fact deserves notice, for we here probably see the combined effects of the bending down from weight and of circumnutation.  The stolon, however, did not circumnutate when it first began to bend down, as may be observed in the present diagram, and as was still more evident in the last case, when a longer portion of the stolon was left unsupported.  On the following day (26th) the stolon moved twice up and twice down, but still continued to fall; in the evening and during the night it travelled from some unknown cause in an oblique direction.]

We see from these three cases that stolons or runners circumnutate in a very complex manner.  The lines generally extend in a vertical plane, and this may probably be attributed to the effect of the weight of the unsupported end of the stolon; but there is always some, and occasionally a considerable, amount of lateral movement.  The circumnutation is so great in amplitude that it may almost be compared with that of climbing plants.  That the stolons are thus aided in passing over obstacles and in winding between the stems of the surrounding plants, the observations above given render almost certain.  If they had not circumnutated, their tips would have been liable to have been doubled up, as often as they met with obstacles in their path; but as it is, they easily avoid them.  This must be a considerable advantage to the plant in spreading from its parent-stock; but we are far from supposing that the power has been gained by the stolons for this purpose, for circumnutation seems to be of universal occurrence with all growing parts; but it is not improbable that the amplitude of the movement may have been specially increased for this purpose. [page 223]

Circumnutation of flower-stems.

We did not think it necessary to make any special observations on the circumnutation of flower-stems, these being axial in their nature, like stems or stolons; but some were incidentally made whilst attending to other subjects, and these we will here briefly give.  A few observations have also been made by other botanists.  These taken together suffice to render it probable that all peduncles and sub-peduncles circumnutate whilst growing.