302 BOTANY PART I 



after they have been cut off, are able to give rise to new plants, in other cases the 

 leaves jxmess this power while still growing on the parent plant. Some ferns 

 afford specially characteristic examples of this (Asplenium decussatum, A. 

 Fabianum, A. bulbiferum, A. viviparum) ; adventitious buds are produced on their 

 laminae, developing into small rooted plants, which then fall off and complete their 

 development (Fig. 239). The adventitious buds of Cystopteris bulbifera take the 

 form of bulbils with small swollen leaves. Adventitious plantlets are frequently 

 formed also on the leaves of Cardamine pratcnsis, and Cardamine amara manifests 

 a similar tendency. One of the best-known examples of such adventitious forma- 

 tions is afforded by the leaves of the tropical Bryophyllum, in the marginal indi-ntu- 

 tions of which the brood plantlets develop in great numbers. Even the force of the 

 wind is sufficient to detach the leaflets of Bryophyllum from the plant. Gemmae 

 are abundantly produced on the thallus of many Hepaticae (Marchantia, Lunularia), 

 and by their continuous growth the gemma cups (Figs. 381, 382) are always kt-pt 

 well filled. 



One of the most interesting cases of adventitious budding is the formation of 



FIG. 230. Afjilenluni Fabianum. A young plant (T), with leaves and roots (W), 

 has sprung from the leaf (J/) of tin- nldi'r plant. 



adventitious embryos in ovules ; in most instances this leads to POLYEMBRYONY, 

 i.e. the presence of several embryos within the one seed. In addition to cases in 

 which the additional embryos cannot be strictly regarded as adventitious but 

 arise by fusion of ovules, by division of the nucellus within the integments, or by 

 the existence of a number of embryo-sacs in one nucellus, normal ovules with only 

 one embryo-sac may produce a number of embryos. In the latter case the embryo- 

 sac may in rare instances (Santalum, Sinninyia) contain two egg-cells, or a 

 dichotomy of the embryo or its suspensor may occur. As a rule the condition is 

 due to adjoining vegetative cells growing into the embryo-sac and there develop- 

 ing like sexually produced embryos. These adventitious embryos, which were 

 first discovered by Strasburger, usually originate from cells of the nucellus (Funkia 

 ovata (Fig. 240), Euonymus latifolia, Citrus aurantium, Nothoscordon fragrans, 

 Mangifera indica, Coelebogyne illicifolia, Clusia alba, Opuntia vulgarc, Euphorbia 

 dulcis, Colchicum autumnale) more rarely from the inner integuments (Allln,i> 

 odorum). In many other plants (Gnetum, Ulmus) embryos may develop from the 

 synergidse or the antipodal cells. At the same time the egg-cell previously 

 existing in the embryo-sac is able to continue its development after fertilisation, 

 but is usually prevented from so doing by the adventitious embryos. The seeds 

 in such cases would no longer contain the products of sexual reproduction, but 



