260 



THE STRUCTURE OF FLOWERS. 





Fig. 57. — Cleistogamous flower-buds of Oxalis 

 Acetosella. (For description, see text.) 



standing the cold and wet summer (1886), tlie plants 

 observed almost invariably bore frnit."* 



As an example of pure cleistogamy I will take Oxalis 

 Acetosella, as having special peculiarities. Mr. Darwin 

 alludes to M. Michalct's description of the cleistogamous 

 flowers of this species,t and adds some observations of his 

 own.J He quotes an observation of Michalet's, that the 

 five shorter stamens are sometimes quite aborted. This fact, 



which I have also ob- 

 served (Fig. 57, d), is 

 quite in keeping with 

 the common process of 

 the reduction of the 

 number or parts of sta- 

 mens in self-fertilising 

 flowers. He also adds this interesting observation : " In one 

 case the tubes, vphich ended in excessively fine points, were 

 seen by me stretching upwards from the lower anthers towards 

 the stigmas, Avhich they had not as yet reached. My plants grew 

 in j)ots, and long after the perfect flowers had withered they 

 produced not only cleistogamic, but a iew minute open flowers, 

 which were in an intermediate condition between the two 

 kinds." This last remark is quite in accordance with the 

 true origin of these flowers, that they are in all cases degra- 

 dations from the conspicuous forms normally characteristic 

 of the species which produce tbera. 



Fig. 57, a, clearly shows that in Oxalis Acetosella the 

 cleistogamous state is simply a flower-bud which has become 

 adapted to self-fertilisation ; and the intermediate conditions 

 alluded to by Mr. Darwin I should suspect were analogous to 



* Journ. Boy. Micr. Soc, 1887, p. 615, and note. See below, pp. 270, 271. 

 t Bull. Soc. Bot. de Fr., rii. (I860), p. 465. 

 X Forms of Flowers, p. 32]. 



