Page:The Journal of Indian Botany.djvu/389



Loeb in 1917 published (l) the results of a series of experiments on curvature induced in cut stems of Bryophyllum calycinum, sus- pended in moist air by threads, one at each end. He found, and published figures in support, that curvature, apparently due to gravity took place, whether leaves were left attached to the plant or not ; but much more strongly so, when a leaf near the apical end was left, than when all but a basal leaf were removed. He also found that roots were formed adventitiously, on that region only where the curvature was a maximum, and that root-formation was strongest, when an apical rather than a basal leaf was left. From this, he drew the conclusion, that special root-forming and curvature-producing substances (hormones) are manufactured in the leaf, and passed downwards, but not upwards, along with the products of assimilation.

The experiments described here, were made to test these facts and deductions on plants growing in Madras. Healthy stems of Bryophyl- lum calycinum were used and straight pieces 6 to 8 inches long were cut, and in every case the apical bud and the first visible internode, were removed to prevent continued growth, as was done by Loeb.

All the leaves were removed except one or both apical leaves or, one or both basal leaves, and the pieces suspended over water in a glass chamber. No particular difference was observed, which could be correlated with the presence of one or both leaves, whether at the base or apex: But those with one or both apical leaves curved more than those with basal. (Plate I, figs 1, 2 and 3) Loeb in explaining this as due to the formation of geotropic hormones in the leaf and their passage always down the stem, appears to have overlooked the possibility of the curvature being induced by weight, for no attempt apparently, was made to eliminate this. We found however, that if a third thread was used to support the piece at its centre of gravity or if the pieces were laid on a flat board (fig. 11) no curvature took place. (Compare in Plate I, figs. 8, 9, 10 with 5, 6, 7.)

In order to support the piece without introducing other factors, we attached to the piece near its centre of gravity a fine thread, which after passing over a pulley, supported a pan, in which weights were