342 SELF-STEEILE PLANTS. CHAP. IX. 



in the genus Passiflora, and with the Vandese amongst 

 Orchids. 



Self-sterility differs much in degree in different 

 plants. In those extraordinary cases in which pollen 

 from the same flower acts on the stigma like a poison, 

 it is almost certain that the plants would never yield a 

 single self-fertilised seed. Other plants, like Corydalis 

 cava, occasionally, though very rarely, produce a few 

 self-fertilised seeds. A large number of species, as 

 may be seen in Table F, are less fertile with their own 

 pollen than with that from another plant ; and lastly, 

 some species are perfectly self-fertile. Even with the 

 individuals of the same species, as just remarked, 

 some are utterly self-sterile, others moderately so, and 

 some perfectly self-fertile. The cause, whatever it may 

 be, which renders many plants more or less sterile 

 with their own pollen, that is, when they are self- 

 fertilised, must be different, at least to a certain extent, 

 from that which determines the difference in height, 

 vigour, and fertility of the seedlings raised from self- 

 fertilised and crossed seeds; for we have already 

 seen that the two classes of cases do not by any means 

 run parallel. This want of parallelism would be 

 intelligible, if it could be shown that self-sterility 

 depended solely on the incapacity of the pollen-tubes 

 to penetrate the stigma of the same flower deeply 

 enough to reach the ovules ; whilst the greater or less 

 vigorous growth of the seedlings no doubt depends on 

 the nature of the contents of the pollen-grains and 

 ovules. Now it is certain that with some plants the 

 stigmatic secretion does not properly excite the pollen- 

 grains, so that the tubes are not properly developed, 

 if the pollen is taken from the same flower. This is 

 the case according to Fritz Muller with Eschscholtzia, 

 for he found that the pollen-tubes did not penetrate 



