PHYSIOLOGY 



279 



and by their continuous growth the gemmEe capsules (Fig. 316, b) are always kept 

 well filled. 



A most remarkable instance of adventitious budding sometimes occurs, in which 

 adventitious buds, which have arisen in the nucellus of the ovule, grow into the 



Fig. 215. — Asplenium Fabiatium. "A young plant (r), with leaves and roots (W), has sprung 

 from the leaf (j¥) of the older plant. 



embryo-sac, and there develop just as if they were embryos ; examples of this 

 phenomenon may be found in Evonymus, Citrus, Funkia (Fig. 216), Coelebogyne. 

 Formerly it was thought that such a 

 polyembeyony was due to the existence 

 of numerous egg -cells in one embryo- 

 sac ; but more thorough investigation 

 has shown, however, that it arises from 

 the vegetative formation of adventi- 

 tious geems. At the same time the egg- 

 cell previously existing in the embryo- 

 sac is able to continue its development 

 after fertilisation, but is usually pre- 

 vented from so doing by the adventi- 

 tious or nucellar embr3'OS. The seeds in 

 such cases would no longer contain the pro- 

 ducts of sexual reproduction, but would be 

 degraded to organs of vegetative multi- 

 plication. The adventitious germs in 

 the polyembryonic seed are, however, so 

 far dependent upon sexual reproduction, 

 that for the most part they only attain 

 their development in case fertilisation has 

 previously taken place ; but in Coelebo- 

 gyne, one of the Australian. Euphorbiaccac, 

 of which usually only female specimens 

 are found in cultivation, the adventitious 

 germs develop without the stimulus of 

 fertilisation. This plant, accordingly, affords another example of apogamy, or of the 

 substitution of a vegetative for a sexual mode of reproduction, such as occurs in 

 different degrees in certain Ferns, Athyrium filix feinina var. eristatum, Aspidium 

 faleatum, Todea africana, and Pteris cretiea. In the last-named example the sexual 



Fig. 216. — Vegetative formation of embryos in 

 Funkia owta (Hosta coerulea) by the budding 

 of the nucellus ; ■//., nucellus with cells in 

 process of forming the rudiments (ae) of the 

 adventitious embryos ; S, synergidai ; E, egg- 

 cell, in the lower figure developing into a sexu- 

 ally-produced embryo. (After Strasbttrger.) 



