In V. nana and elatior only two of the stamens properly bear anthers, but sometimes one or even two of the others are thus provided. Lastly, in V. canina never more than two of the stamens, as far as I have seen, bear anthers; the petals are much more reduced than in V. hirta, and according to D. Muller are sometimes quite absent.
Oxalis acetosella.
The existence of cleistogamic flowers on this plant was discovered by Michalet. (8/9. 'Bulletin Soc. Bot. de France' tome 7 1860 page 465.) They have been fully described by Von Mohl, and I can add hardly anything to his description. In my specimens the anthers of the five longer stamens were nearly on a level with the stigmas; whilst the smaller and less plainly bilobed anthers of the five shorter stamens stood considerably below the stigmas, so that their tubes had to travel some way upwards. According to Michalet these latter anthers are sometimes quite aborted. In one case the tubes, which ended in excessively fine points, were seen by me stretching upwards from the lower anthers towards the stigmas, which they had not as yet reached. My plants grew in pots, and long after the perfect flowers had withered they produced not only cleistogamic but a few minute open flowers, which were in an intermediate condition between the two kinds. In one of these the pollen-tubes from the lower anthers had reached the stigmas, though the flower was open. The footstalks of the cleistogamic flowers are much shorter than those of the perfect flowers, and are so much bowed downwards that they tend, according to Von Mohl, to bury themselves in the moss and dead leaves on the ground. Michalet also says that they are often hypogean. In order to ascertain the number of seeds produced by these flowers, I marked eight of them; two failed, one cast its seed abroad, and the remaining five contained on an average 10.0 seeds per capsule. This is rather above the average 9.2, which eleven capsules from perfect flowers fertilised with their own pollen yielded, and considerably above the average 7.9, from the capsules of perfect flowers fertilised with pollen from another plant; but this latter result must, I think, have been accidental.
Hildebrand, whilst searching various Herbaria, observed that many other species of Oxalis besides O. acetosella produce cleistogamic flowers (8/10. 'Monatsbericht der Akad. der Wiss. zu Berlin' 1866 page 369.); and I hear from him that this is the case with the heterostyled trimorphic O. incarnata from the Cape of Good Hope.
Oxalis (Biophytum) sensitiva.
This plant is ranked by many botanists as a distinct genus, but as a sub-genus by Bentham and Hooker. Many of the early flowers on a mid-styled plant in my hothouse did not open properly, and were in an intermediate condition between cleistogamic and perfect. Their petals varied from a rudiment to about half their proper size; nevertheless they produced capsules. I attributed their state to unfavourable conditions, for later in the season fully expanded flowers of the proper size appeared. But Mr. Thwaites afterwards sent me from Ceylon a number of long-styled, mid-styled, and short-styled flower-stalks preserved in spirits; and on the same stalks with the perfect flowers, some of which were fully expanded and others still in bud, there were small bud-like bodies containing mature pollen, but with their calyces closed. These cleistogamic flowers do not differ much in structure from the perfect ones of the corresponding form, with the exception that their petals are reduced to extremely minute, barely visible scales, which adhere firmly to the rounded bases of the shorter stamens. Their stigmas are much less papillose, and smaller in about the ratio of 13 to 20 divisions of the micrometer, as measured transversely from apex to apex, than the stigmas of the perfect flowers. The styles are furrowed longitudinally, and are clothed with simple as well as glandular hairs, but only in the cleistogamic flowers produced by the long- styled and mid-styled forms. The anthers of the longer stamens are a little smaller than the corresponding ones of the perfect flowers, in about the ratio of 11 to 14.