Fertilisation of Polygala

Abstract

HAS the method of fertilisation of the milkwort, Polygala vulgaris, yet been described? It presents one of the most beautiful contrivances with which I have hitherto met for securing a cross through the agency of insects. The corolla consists of five petals united into one piece and folded into the form of a two-lipped tube, the upper lip of which is formed by the over-lapping edges of the corolla; while the lower lip is a somewhat cup-shaped appendage (c), furnished with a “beard” of gland-like bodies (b), and opening in front by a narrow, vertical slit. The filaments of the stamens are united throughout the greater part of their length with the corolla, but expand within the cup of the lower lip into a two-lobed membrane, crowned by the anthers (a). The pistil has two stigmas, one of which (s) is placed at right angles to the upper side of the style and is perfect, while the other (s′) is transformed into a spoon-shaped, petaloid prolongation of the pistil, reaching to the opening of the lower lip of the corolla, and dividing the interior of the flower into two chambers, in the lower of which are the stamens, thus completely separated from the true stigma. The entrance to the flower, below the style and in front of the stamens, is closed by hairs pointing outwards from the flower and meeting in front, on the mouse-trap principle; but a narrow passage is left open above the petaloid stigma, and is perhaps capable of a slight distension from the flexibility of the overlapping petals. On each side of the interior of the tube of the corolla, above the style and just behind the true stigma, is a group of strong, white hairs (h), pointing down the tube of the corolla, and meeting above the style. If we now suppose a small insect to light upon the “beard” of the flower, it is prevented from immediate entrance by the projecting hairs, but soon finds the narrow passage leading over the stigma into the upper chamber. It is prevented by the hairs in the tube of the corolla from returning by the same path, and is obliged to crawl out through the lower chamber and over the stamens; pollen from which it will, by a repetition of the same process, convey to the stigma of the flower next visited.