The causes of the bud eccentricity and the large divergence angles between leaves in Cucurbitaceae

Abstract

In Cucurbitaceae the axillary buds are eccentric, being much displaced in the anodic or up-hill direction of the genetic leaf spiral: also the angles between successive leaves are unusually large. In the young seedling the leaf spiral and the bud eccentricity both start with the third leaf from the base. If a radial vertical cut of some little depth is made in the apex at the kathodic side of P 1 , the youngest leaf primordium, or of I 1 , the next primordium due to arise, the bud of the P 1 or I 1 is often median or nearly so. Also, if P 1 or I 1 is isolated from the apex with a vertical cut made in the tangential direction and is then extirpated, the bud of the next primordium, I 1 or I 2 , is often subeccentric or nearly median. But shaving down the upstanding part of P 1 flush with the apex, or preventing with shallow cuts P 1 or its bud or both from developing does not diminish the eccentricity of any subsequent bud. It is concluded that an axillary bud is made anodic by some repelling influence, perhaps a hormone, which originates from an older leaf primordium and travels in the anodic direction of the leaf spiral. This influence is interrupted by a cut of moderate depth in the apex, but not by a very shallow cut. The changes in the positions of certain younger leaves due to the formation of median or nearly median buds after these operations strongly support an explanation of the large angles between successive leaves based on a space-filling theory of phyllotaxis. This explanation is that the position in which any leaf n is determined is displaced in the anodic direction by the anodic axillary bud of leaf n — 3, which encroaches from the kathodic side upon the space available for n between n — 3 and n — 2. When the bud of n — 3 is median or nearly so, the position of n is much less far anodic. The repelling influence that makes the buds anodic is further discussed in relation to the course of the conducting strands and other relevant facts.