Art. XLI.—Note on the Cleistogamic Flowers of Melicope simplex.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 24, 1891, Page 416

Art. XLI.—Note on the Cleistogamic Flowers of Melicope simplex. By Geo. M. Thomson, F.L.S. [Read before the Otago Institute, 10th November, 1891.] Melicope simplex is a common shrub in Otago, and in the east coast districts produces its small greenish-white flowers in October and November, but in the interior a month or so later. These average about ⅜in. in diameter, and are fragrant. I have not, however, detected any nectar in them, nor do I know how they are fertilised. In a paper on “Fertilisation of Flowering-plants” (Trans. N.Z. Inst., vol. xiii., p. 257) I suggested that these flowers were probably fertilised by “the numerous small Diptera which so commonly frequent the edge of the bush,” but I have never been able to verify this surmise. Though sometimes quite hermaphrodite, it is frequently the case that they are so in structure only, being unisexual in function; while others again are strictly diœcious, the male form having no pistil, and the female flowers having stamens with more or less imperfect anthers, and little or no pollen. This feature is remarkably common among New Zealand flowering-plants, and in several species it is possible to find transition stages between perfectly-hermaphrodite and perfectly-diœcious plants. But truly cleistogamic flowers, self-fertilised and remaining quite closed until the fruit is considerably developed, have hitherto been recorded from very few species.

When at Lake Wanaka some two years ago I landed on Pigeon Island, on which Melicope simplex is abundant, and found that all the plants were covered with closed flowers, which were freely setting seed. I collected a number of these flowers, but had not the opportunity till this spring of comparing them with the ordinary form. In the normally flowered specimens of this species growing near Dunedin the 4 petals spread widely open. The 8 stamens are in two whorls, those of the outer or sepaline whorl being distinctly longer than those of the inner or petaline whorl. The 4 carpels are crowded closely together in flower, and their styles are united into one, which is crowned by a capitate stigma showing no trace of divisions; but as the fruit ripens the carpels tend to grow away from one another. Of the two ovules present in each carpel, only one ripens, as a rule, the other remaining undeveloped. A. Normal flower B. Cleistogamic flower s. Sepal. p. Petal. s.s. Stamens of the outer or sepaline whorl, p.s. Stamens of the inner or petaline whorl. In the cleistogamic flowers of my Lake Wanaka specimens the sepals were normally produced. The petals are perhaps shorter, but do not otherwise differ in form from those of the ordinary flowers. They do not open, however, their edges remaining in close contact until forced apart by the expansion of the ovaries. The outer or sepaline stamens are either reduced to minute pointed rudiments of filaments, or they bear small—apparently aborted—anthers on very slender filaments, which stand close up to the base of the carpels, and are pressed in between the lobes. On the other hand, the stamens of the inner whorl have large anthers on very short filaments. The carpels, though standing close together, are quite distinct from one another throughout their whole length, and, instead of having one style common to all 4 carpels, there are 4 separate or nearly - separate styles. In other respects the carpels develope normally, producing, at most, only one mature seed. Fertilisation must either take place at a very early stage in

the development of the flowers, or, which is more probable, the pollen must be shed early into the pendulous flowers, and, being caught in the sac made by the closed petals, is thus brought into contact with the stigmas. In all the flowers examined by me the ovaries stood always too high to receive pollen directly on their stigmas, unless it reached them in the manner suggested. No definite cause has been assigned for the occurrence of cleistogamic flowers in plants. The Rev. George Henslow, in “The Origin of Floral Structures” (p. 262), gives several examples of species which produce normal flowers in one district or country and cleistogamic flowers in another, showing that the change appears to be due to variations in climate or soil, &c.; but no law can be adduced from the examples given. That the occurrence of this phenomenon depends largely on the supply of moisture is suggested by the following fact: Some plants of violet (cultivated forms of Viola odorata) growing in my garden, in a border against a wall—a position in which they were subjected to great heat and where they got very little moisture—were found covered with cleistogamic flowers, and bore very numerous seed-capsules, although they produced no conspicuous flowers. Other violets of the same variety, growing only a few feet away, but in a border where they were exposed to the weather and got abundance of moisture, had abundance of normal but no cleistogamic flowers. I can give no explanation of the specimens of Melicope from Lake Wanaka, as I have no record of the soil or climatic conditions of Pigeon Island; but the profusion of flowers, not one of which was open, and of maturing fruit, arrested my attention at once, and I noticed that all the plants seen by me were so covered with these peculiar self-fertilised closed flowers.