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Art. LX.—Notes on Cleistogamic Flowers of the Genus Viola.
[Read before the Otago Institute, 14th May, 1878.]
It is a well-known fact that, owing to the poverty of insect life in these islands, the number of entomophilous plants, i.e., those requiring insect aid in securing fertilization, is small in comparison with most other parts of the world. Hence, also, the comparative want of gaily-coloured flowers, and the prevalence of white, green, and inconspicuous flowers. As every fact bearing on the question of fertilization of flowers gives us additional insight into the relations of the indigenous fauna and flora, I make no further apology for communicating the following notes to the Institute.
There are many plants which produce two kinds of hermaphrodite flowers, viz., tolerably large and conspicuous flowers, fitted for cross-fertilization by means of insects, and small, closed ones, more or less depauperated, and sometimes produced underground, fitted only for self-fertilization. These last are known as Cleistagamic (Gr. Kleistos, closed; gamos, union). In Darwin's latest botanical work, “On Different Forms of Flowers,” there is given a list of fifty-five genera, certain species of which produce these flowers. Of the genus Viola, fifteen species are named, which produce,
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more or less completely, cleistogamic flowers. V. tricolor, the parent plant of our garden pansy, does not produce them, and we find in the above list a gradation in the amount of depauperation, which appears to reach its maximum in the Indian V. nana. This species, though producing perfect flowers in its native habitat in the Sikkim Terai, produced only cleistogamic flowers in Calcutta, and in Mr. Darwin's greenhouse, and this for many successive seasons.
Three species of Viola have been described as occurring in New Zealand. Of these I have not seen V. Lyalli, but have examined the other two. The presence of cleistogamic flowers on these plants has long been known, but their structure has not been minutely described, and most people take them for buds.
Viola filicaulis, Hook. f.
This species bears exceedingly variable flowers, some being only slightly depauperated, while others are completely closed. I am strongly inclined to think that those plants which grow in open, sunny spots, produce more of the conspicuous flowers than those growing in hidden and out-of-the-way corners. Clumps of the plant were in many cases gathered from deep clefts among rocks, and these were found in most cases to be covered with cleistogamic flowers, but to have few or none of the conspicuous ones.
The ordinary flowers of this plant are produced on slender peduncles from three to six inches in length.
The petals, which are about twice as long as the sepals, are white or pale blue, elegantly streaked with brown and yellow; the lower one being furnished with a very short, obtuse spur. The stamens are well developed, with a thin, broad connective, which extends considerably above the anther lobes, and the two lower stamens are furnished with short, truncate spurs. The style is long and curved, and terminated by a quadrangular stigmatic aperture.
1, a. Spurred stamen from conspicuous flower of V. vilicaulis x 25.
1, b. Pollen grains from the same x 225.
The cleistogamic flowers are borne on curved peduncles, very close to the root, from ¼ to 1 inch in length. The flowers themselves are very small, seldom exceeding a quarter of an inch in length. The sepals are similar in form and development to those of the ordinary flowers. The petals are shorter than and included in the sepals, and are all regular in shape. The stamens appear to be all represented, but none have the spur. The filaments are narrow, not extended laterally, but prolonged shortly above the anthers to a short, acute hood. The anthers are applied closely to the pistil, and the pollen grains,
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while still in the anther cells, may be seen emitting their tubes to the stigma. The style is nearly as long as that of the conspicuous flowers, but in order to bring the stigma within reach of the pollen, it lies coiled and twisted on the summit of the ovary. In some of the flowers all the stamens were reduced to mere filaments except one; in others there occurs one stamen with both anthers; another with a single anther, the rest being represented by filaments; while others again had the rudiments of anthers, but no pollen. The size of the pollen grains varied some-what in the two kinds of flowers. In the large, conspicuous flowers, the grains appeared uniform in size and in shape, and averaged about 1/600th of an inch in length; in the cleistogamic flowers, the larger grains were similar in shape and length to those of the larger flowers, while the majority were rounder and thicker, and not more than 1/1000th of an inch in length.
2, a. Stamens from cleistogamic flowers of V. filicaulis x 25.
2, b. Pollen grains from the same x 225.
Viola cunninghamii, Hook. f.
This species, as a rule, flowers earlier than the preceding. When examined by me during the month of January, it bore abundance of cleistogamic flowers, while the empty valves of their capsules showed that the other flowers had already ripened and shed their seed. The ordinary flowers are very similar to those of V. filicaulis, and are produced on peduncles of similar length. The cleistogamic flowers are produced on very short peduncles, which lengthen in fruit to ½ or. ¾ of an inch. All the parts in these flowers are very small. The sepals are all present, and of the normal form, but the petals are wanting; their disappearance seems to be complete, as there was no trace of their presence.
The stamens are very feebly developed. Two of them have both their anthers developed, but these are very small and do not contain much pollen. The filament is narrow below, but has its upper part extended into a hood. These two stamens are placed on opposite sides of the pistil, and their hoods completely overlap and enclose it, their anthers being in contact with the short, obtuse stigma.
In all the flowers examined by me, there seemed no departure from this type of structure, nor was there the variability which I noticed in the cleistogamic flowers of V. filicaulis.
3, a. Hooded stamen from cleistogamic flower of V. cunninghamii x 25.
3, b. Rudimentary stamen from the same x 25.