Experiments on growth and inhibition. Part II.-New phenomena of inhibition

Abstract

By decapitating young pea seedlings in the epicotyl, plants can be prepared with 2 shoots springing from the axils of the cotyledons. In such plants, if 1 of the shoots has its leaves removed until only those 1 mm. long or less remain, it is rapidly arrested in growth and finally killed. This effect must be due to inhibition coming from the intact shoot, for if similar single shoots are similarly defoliated they grow on rapidly and indefinitely. It makes no appreciable difference to these results whether the plants are grown in good light or, for the first 12 days after the operation, in very dim light. It is concluded that the effective part of the defoliation is the removal of the rapidly developing young leaves, which are also the inhibiting members, and not the removal of the expanded photosynthetic leaves. Similar results are produced, though less rapidly, by darkening a shoot. If another shoot is present on the same plant, the darkened shoot is arrested and killed, but if not it grows on rapidly. These results make it probable that the inhibiting influence that comes from the developing leaves of a shoot is of a polar nature, so that 2 such influences coming from 2 cotyledonary shoots from opposite directions counteract one another. If 1 of the shoots is defoliated or otherwise weakened, the influence coming from the other shoot travels up into it, and arrests and kills it. The influence coming from the developing leaves kills (directly or indirectly) those shoots or parts of shoots which are not in the line between developing leaves and roots, and in which it travels acropetally.