242 THE PHYSIOLOGY OF PLANTS 



doubtful. The greatest certainty exists, of course, in those 

 cases in which specialists are occupied with a single genus or 

 species. Then the repetition of former results may take the 

 place of experiments performed with all the precautions which 

 ought always to be taken when a cross-fertilisation can only- 

 be attempted once. 



The Orchids seem to be a group especially welL adapted for 

 such experimental work. They are now-a-days the fashion- 

 able flowers, and their fertilisation is carried out by gardeners 

 with great care ; besides which, their flowers can only be 

 fertilised by very special insects, which do not exist in this 

 country. Fertilisation by wind or insects while the plants 

 were under cultivation in Europe has only been recorded for 

 Phajus grandifolius, Aerides ajfflne, Vanda Roxburghii, Lcelia 

 cinnabarina, Oypripedium Schlimii, javanicum, virens, Bulleni- 

 anum. The signs which indicate that a flower is really 

 fertilised do not. always appear in the same way or after the 

 same lapse of time. Gardeners must therefore not be too 

 rash in drawing conclusions from the non-appearance of 

 certain signs. The most well known of these is the swelling 

 of the ovary, which takes place in some cases very rapidly, 

 as has been observed in. Oncidium (Fapilio Forhesii), Odonto- 

 glossum, Lcelia, Cattleya, Phalcenopsis, &c. Other orchids close 

 their flowers the day after the fertilisation has been effected. 

 Cypripediiim remains fresh for a considerable period, and its 

 ovary, shows no sign of swelling. 



On the other hand, it must be pointed out that changes 

 which generally take place after pollination are not always a 

 sure sign that fertilisation has been effected. Of these, the 

 most obvious one is the growth of the pollen tube down the 

 style of another species or genus. Strasburger, for instance, 

 mentions that the pollen tube from a pollen grain of Lathyrus 

 montanus grew down into the ovarian cavity of Convallaria 

 latifolia, ■while tubes from the pollen of Agapanthus grew deep 

 down into the style of Achimenes grandiflora (but not vice 

 versa). The pollen tubes of Fritillaria persica are said to 

 grow into the ovarian cavity of orchids, and to cause the 

 development and swelling of its oyules. 



We have no rules of general application for hybridisation. 



