Art. XLIII.—On the Pollination of the Puriri (Vitex lucens, T. Kirk).
[Read before the Auckland Institute, 4th July, 1904.]
The puriri is a well-known beautiful and valuable tree that grows throughout the lowland parts of the Auckland Province, and extends as far south as Mahia Peninsula on the east coast and Cape Egmont on the west.
Its chief time of flowering is in the winter, though stray flowers may be found at most seasons of the year, and trees may always be found in full bloom during the months of May, June, July, and August. The flowering season of single trees often extends over two months or more, and it is no uncommon thing to see full-grown fruit and young flowers on the same branch, and even on the same panicle.
The flowers grow in spreading flattened axillary panicles among the upper or younger leaves. Those borne in a panicle vary in number from four to twelve, and are supported on rather slender but fairly rigid flower-stalks. The flowers are of fair size, being about an inch in length and nearly an inch wide in front.
The calyx is short and cup-shaped. The corolla, which is pink or more usually dull-red in colour, is tubular and irregular, with a four-lobed limb. The upper lobe or lip is comparatively short, slightly arched, and either entire or bifid. The lower lip is much larger and broader, strongly deflexed, and trifid.
The stamens, four in number, spring from the lower part of the corolla-tube, and have long filaments. The bases of the filaments and the parts of the corolla between their points of insertion are densely clothed with a felted mass of long hairs that completely blocks the tube of the corolla leading down to the ovary.
The pistil consists of a short subconical ovary, situated below the level of the bases of the filaments, and of a long and fairly stout polished style, terminating in two (rarely in three) short divergent style-branches. The stigmatic surfaces are confined to the extreme tips of the style-branches, and are not larger than the head of a pin. Where the style joins the ovary there is a shallow constriction, and it is this groove that secretes most, if not all, of the abundant nectar that bathes the ovary, and indeed generally fills the entire space between it and the plug of hairs that blocks the corolla-tube.
Now, if the development of the flowers be carefully watched, a number of curious and interesting phenomena will be observed. Before the flower-bud opens the corolla has nearly reached its full size, and the flower lies in a slightly drooping position, or with its axis horizontal. The front of the corolla-tube is closed by the infolding of the corolla-lobes. The superior lobe lies outermost, the lateral lobes of the lower lip lie within this, and the strongly incurved inferior lobe lies innermost. The stamens are already full-grown. The tips of the filaments are sharply curved downwards, and the anthers, already beginning to dehisce and shed their pollen, are held within the concave infolding of the lowermost lobe of the under lip.
The flower opens by the successive bending back of the lobes of the corolla already mentioned. The deflexed filaments are not elastic, and seem to take little, and probably no, part in hastening the opening of the floral-box containing them. The anthers are now ripe, and the pollen-sacs are gradually everted so that most of the pollen falls, or is blown or is brushed away while the anthers stand in the axis of the corolla-tube. In the course of a day or two the filaments straighten themselves out, and finally lie along the upper internal surface of the corolla-tube, and are closely appressed to it, eventually projecting a little beyond the upper lip. The pollen meanwhile has all been shed, and the anthers are shrivelled and withered.
When the corolla has fully opened the secretion of nectar begins, but it is scanty at first. The style at this stage is little more than half-grown, and lies against the upper part of the corolla-tube, between the two pairs of filaments. When the corolla is fully expanded the style begins to elongate, and in two or three days, when the filaments have completed their straightening, it has grown as long as the stamens. When nearly full-grown its tips begin to curve forwards towards the axis of the flower, and ere long the style-branches open back in the axis of the flower and develop their small terminal stigmatic surfaces ready to receive any grains of pollen that may be brought in contact with them. Throughout this development of the style the secretion of nectar continues to be most abundant, and drops of it will gradually fall out of the corolla-tube if the branches are shaken. The secretion generally continues until the corolla begins to wither.
These are the facts disclosed by careful observation. We see at once that the pollination of the pistil of a flower by pollen from the anthers of the same flower is practically impossible, for the pollen is matured and shed long before the pistil is full-grown or ready for pollination. The movements by which the anthers, and after a few days' interval the style-branches, are
placed and kept for a considerable time in the axis of the corolla-tube are evidently designed to bring about pollination of the pistil by the pollen of some other flower. That other flower may be on the same tree or may be on another. Whether the pollen from another tree is prepotent over that of other flowers on the same tree I am unable to say, and only an elaborate inquiry could decide. It is, however, obvious that, owing to the prolonged period of flowering of single trees, there are abundant opportunities of pollination with the pollen of other flowers on the same tree.
Let us now consider what may be the agents that effect pollination. There is nothing to suggest that the wind blows the pollen from flower to flower or from tree to tree, for all the structural features that characterize wind-fertilised flowers are absent. Though the secretion of nectar is both abundant and long-continued, flying insects do not frequent the flowers; and indeed the store of nectar is so carefully protected by the natural plug of matted hairs obstructing the corolla-tube that insects could reach it only by biting through the base of the corolla-tube, and this I have never known to occur. There is no doubt that pollination is effected exclusively by small birds. These constantly visit the flowers, hang on the rigid leaf-stalks or flower-stalks, and insert their bills into the corolla-tube to suck the nectar. In sucking the sweet juice the tui may be seen grasping a flower in one foot and turning it round into a more convenient position. In passing from flower to flower the birds cannot avoid bringing pollen from young flowers to older ones, and so effecting pollination. That the arrangement answers its purpose is shown by the fairly abundant fruit which the puriri bears even in the neighbourhood of cities, where native birds are now scarce.
The mechanism for securing pollination is much more complete in the puriri than in Rhabdothamnus solandri, the store of nectar is much more copious, and is secreted for a longer time, while the provision for preventing insects from plundering it is most complete.
Altogether the arrangements herein described constitute one of the most interesting and remarkable adaptations of floral structure to the habits of honey-sucking birds that have so far-been detected in our flora.