| OCR Text |
Show 336 CONCLUDING REMARKS CHAP. VIII. by no means the case ; for various parts have been specially modified, so as to aid ~n the self-fertilisation of the flowers and as a protection to the pollen ; for instance the hook-shaped pistil in Viola and in son;te other o-~nera by which the stigma is brought close to the 5 fertiie anthers,-the rucli1nentary corolla of Specularia modified into a perfectly closed tympanum, and the sheath of Monochoria 1nodified into a closed sack,-the excessively thin coats of the pollen-grains, -the anthers :not being all equally aborted, and other such' cases. Moreover Mr. Bennett has shown that the buds of the cleistoga1nic and . perfect flowers of Impatiens differ at a very early period of gr~wth. The degree to which many of the most Important organs in these degraded flowers have be_en reduced or even wholly obliterated, is one of theu most _r~markable peculiarities, reminding us of 1nany p~rasitlC animals. In some cases only a single anther Is left, and this contains but few pollen;.grains of diminished size · in other cases the stigma has elisa ppeared, leaving a simple open passage into the ovariu~ .. It is also interesting to note the complete loss o~ tnfhng points in the structure or functions of certain parts, which though of service to the perfect flowers, ar~ of none to the cleistogamic ; for instance the collectmg hairs on the pistil of Specularia, the glands . on the calyx of the Malpighiacem, the nectar-secreting ~ppendages to the lower stamens of Viola, the secretiOn of nectar by other parts, the emission of a sweet .odour, and apparently the elasticity of the valves In the buried capsules of Viola odorata. We here see, as throughout nature, t h at as soon a s any part or character becomes superfluous it tends sooner or later to disappear. h Another peculiarity 1n these flowers is that t e CHAP. VIII. ON CLEISTOGAMIC FLO.WERS. pollen-grains generally emit their tubes whilst still enclosed within the anthers; but this is not o r - markable a fact as was formerly thought, wh n th case of Asclepias was alone known.* It is, however, a wonderful sight to behold the tubes directing theinselves in a straight line to the stig1na, when thi is at some little distance from the anthers. As soon as they reach the stigma or the open passao· -• leading into the ovarium, no doubt they penetrate it, guided by the same means, whatever these may be, as in the case of ordinary flowers. I thought that they might be guided by the avoidance of light: some pollen -grains of a willow were therefore immersed in an extremely weak solution of honey, and the vessel was placed so that the light entered only in one direction, laterally or fro1n below or from above, but the long tubes were in each case protruded in every possible direction. As cleistogamic flowers are completely closed they are necessarily self-fertilised, not to mention the absence of any attraction to insects; and they thus differ widely from the great majority of ordinary flowers. Delpino believes t that cleistogamic flowers have been developed in order to ensure the production of seeds under cli1natic or other conditions which tend * The case of Asclepias was described by R. Brown. Baillon asserts ('Adansonia,' tom. ii. 1862, p. 58) that with many plants the tubes are emitted from pollengrains which have not come into contact with the stigma; and that they may be seen advancing horizontally through the air towards the stigma. I have observed the e1nission of the tubes from the pollen-masses whilst still within the anthers, in three widely distinct Orchidean genera: namely Aceras, Mal axis, and N eottia : see ' The Various Contrivances by which Orchids are Fertilised,' 2nd edit. .P· 258. t 'Sull' Opera la Distribuzione dei Ses~i neUe Piante,' 18u7, p. 30. z |
