OCR Text |
Show 182 HETEROSTYLED TRIMORPHIC l)LANTS. CHAP. IV. quite fertile with its own pollen when insects are excluded. The stigmas of 0. striata and of another hmnostyled species, viz. 0. tropmoloides, commonly stand on a level with the upper anthers, and both these species are likewise quite fertile when insects are excluded. With respect to 0. acetosella, Hildebrand says that in all the many specimens exa1nined by him the pistil exceeded the longer stamens in length. I procured 108 flowers from the same number of plants growing in three distant parts of England; of these 86 had their stigmas projecting considerably above, whilst 22 had them nearly on a level with the upper anthers. In one lot of 17 flowers from the same wood, the stigmas in every flower projected fully as 1nuch above the upper anthers as these stood above the lower anthers. So that these plants n1ight fairly be co1npared with the long-sty led form of a heterosty led species ; and I at :first thought that 0. acetosella was trimorphic. But the case is one merely of great variability. The pollen-grains from the two sets of anthers, as observed by Hildebrand and n1yself, do not differ in dian1eter. I fertilised twelve flowers on several plants with pollen from a distinct plant, choosing those with pistils of a different length; and 10 of these (i.e. 83 per cent.) produced capsules, which contained on an average 7·9 seeds. Fourteen flowers were fertilised with their own pollen, and 11 of these (i.e. 79 per cent.) yielded capsules, containing a larger average of seed, namely 9·2. These plants, therefore, in function show not the least sign of being heterostyled. I may add that 18 flowers protected by a net were left to fertilise themselyes, and only 10 of these (i.e. 55 per cent.) yielded capsules, which contained on an average only 6·3 seeds. So that the access of insects, or artificial aid in placing pollen on the stigma, increases the fertility of the CHAP. IV. PONTEDERIA. 183 flowers; and I found that this applied especially to those having shorter pistils. It should be remembered that the flowers hang downwards, so that those with short pistils would be the least likely to receive their own pollen, unless they were aided in some manner. Finally, as Hildebrand has remarked, there is no evidence that any of the heterostyled species of Oxalis are tending towards a dioocious condition, as Zuccarini and Lindley inferred from the differences in the reproductive organs of the three forms, the meaning of which they did not understand. PONTEDERIA [ SP. ?] (PONTEDERIACE1E.) Fritz Muller found this aquatic plant, which is allied to the Liliacece, growing in the greatest profusion on the banks of a river in Southern Brazil.* But only two forms were found, the flowers of which include three long and three short stamens. The pistil of the long-styled form, in two dried flowers which were sent me, was in length as 100 to 32, and its stigma as 100 to 80, compared with the sa1ne organs in the shortstyled form. The long-styled stigma projects ~onsiderably above the upper anthers of the same flower, and stands on a level with the upper ones of the short-sty led form. In the latter the stigma is seated beneath both its own sets of anthers, and is on a level with the anthers of the shorter sta1nens in the long-sty led form. The anthers of the longer stamens of the short-sty led form are to those of the shorter stamens of the long-sty led form as 100 to 88 in length. The pollen-grains distended d *"Ueber den Tri'morph1'smus Z e1' ts c h r1' ft.,' & c., B an d 6, 1871 , er Pontederien "; 'J enaische p. 74. |