PREPOTENCY OF POLLEN. 407 



In agreement with Corydalis stand numerous species of Fumaria, and a great 

 number of Papilionaceae, especially those whose flowers possess a piston-apparatus 

 (cf. p. 260). P^sum and Ervum, Lotus and Melilotus, the various species of Tri- 

 fohum, almost all of them, when unvisited by insects, ripen seed, only a few species 

 here and there being infertile when dependent upon their own resources. Thus we 

 may say that when the stigma has to choose between own and foreign pollen, the 

 latter probably gets the preference, though, when own pollen alone is present, it is 

 adequate for fertility. 



A similar condition obtains amongst the Scabiouses (Scabiosa) also. Their 

 flowers are hermaphrodite and protandrous, and united into heads. At the time 

 when the anthers dehisce, the pollen remains hanging to the stigmas, although these 

 are not as yet actually receptive. For the time being, this pollen is without effect 

 By the visits of insects a portion of this pollen is removed and replaced by foreign 

 pollen, which is ultimately, on the maturing of the stigmas, probably preferred to 

 the own pollen. In the absence of insects, however, the flowers are undoubtedly 

 fertile to their own pollen. 



Likewise, in many Labiates (e.g. Leonurus heterophyllus) and Scrophulariace 

 (e.g. Linaria littoralis and minor), has essentially the same state of affairs been 

 shown to exist. One more instance only need be described, that of a Catchfly (Silene 

 noctiflora). This plant opens its flowers at about seven in the evening. If, however, 

 the flowers be opened artificially a little earlier, at about six p.m., it is found that 

 all the anthers have already dehisced, and that the delicate stigmatic papillae are 

 already dusted with the pollen from the five short stamens. Thus, already in the 

 bud, autogamy has taken place in a manner not unlike cleistogamy. As the flowers 

 open in spite of this, it can only be on the chance of crepuscular or nocturnal moths 

 visiting them and bringing foreign pollen. The flowers of this Catchfly are not very 

 eagerly sought after by insects, still now and again a Plusia or other owlet moth 

 may be seen flitting from flower to flower, sucking honey and bringing and taking 

 pollen. Thus, again, a selection of pollen by the stigma probably occurs with 

 preference for the foreign; otherwise, why need these flowers open at all since the 

 stigmas are already coated in the bud with own pollen? In the absence of insects 

 the own pollen will be potent and lead to seed-production. In wet, cold weather 

 also, when the flowers do not open at all, the ovules ripen into seeds, no doubt 

 impregnated by their own pollen. In all the cases enumerated the behaviour is 

 essentially the same, in the early stages of flowering the opportunity is given for 

 cross-pollination by insects, but, wanting this, autogamy or self-pollination ensues. 



We may now pass on to speak of the germination of the pollen-grain upon the 

 stigma and of the development of the pollen-tube. The pollen is at this stage 

 influenced by the receptive stigma. There would appear to be a taking up of fluid 

 matter by the grain, though its exact nature has not been accurately determined.. 

 Since, however, pollen-grains germinate readily in a 3-per-cent sugar-solution, it is 

 extremely probable that sugar is an important component of this stigmatic fluid. 



The first demonstrable stage in the production of the pollen-tube is the pushing: 



