776 PART IV. THE PHYSIOLOGY OF PLANTS. 



susceptible of fertilisation by the male gametes of the species A. 

 Bat this is by no means always the case. Thus Mirabilis Julapa 

 can be fertilised by the pollen of M. longiflora, but the converse 

 crossing has not yet been effected ; similarly the cross Nymplidea 

 ccerulea ? x N. capensis <$ has taken place, but not the cross N. 

 capensis ? x N. coerulea ; and th e cross Fucus vesiculosus ? x F. 

 serratus <, but not F. serratus ? x F. vesiculosus $. It also 

 happens that even -when hybridisation is reciprocal, the one union 

 (A ? x B $ ) is more fertile than the other (B ? x A $ ) ; thus 

 the cross Dianthus barbatus ? x D. superbus is more fertile 

 than the cross D. superbus ? x D barbatus <$ ; and again Digitalis 

 lutea $ x D. ambigua $ than D. ambigua ? x D. lutea . 



These cases of non-reciprocal, or of imperfectly reciprocal hybridisation, seem 

 to be at variance with the statement that fertile hybridisation is dependent upon 

 sexual affiuity ; for if this be true, why is it that the cross A ? x B $ should 

 succeed, but not the cross B $ x A <J , the sexual affinity being necessarily the 

 same in both cases ? The failure of the cross in these cases may be ascribed 

 to purely mecbanical causes ; for instance, whilst MiraHlis Jala-pa is fertilised 

 by the pollen of M. longiftnra, M. longiflora cannot be fertilised by the pollen 

 of M. Jalapa, probably because the pollen-tubes of the latter are unable to 

 grow long enough to rrach from the stigma to the ovule of the former species. 

 All these cases require further investigation. 



Effects of the Sexual Process. The sexual process is not always 

 limited in its effect to the production of a spore which will give rise 

 to a new individual. For instance, when the female cell is borne 

 by the parent at the time of fertilisation, the act of fertilisation in- 

 duces a more or less marked growth and change in the adjacent 

 organs and tissues of the parent, leading to the formation of a 

 fruit (see pp. 88, 528). But further than this, it has been occasion- 

 ally observed in Phanerogams that the constitution of the mother- 

 plant is somewhat modified in consequence of pollination by an- 

 other form. For instance, in the Maize, whilst the grains in the 

 fruit-spike of the mother- plant usually have the colouration peculiar 

 to the mother, from whatever source the pollen may have been 

 derived, it sometimes happens that when a plant whose grains are 

 normally yellow or white is pollinated from another variety whose 

 grains are brown or blue, the resulting grains are brown or blue. 

 It is well known that, in Orchids, the pollination of the stigma 

 acts as a stimulus to the development of the ovary and ovules : 

 and in some cases it appears that even fertile seeds may be pro- 

 duced simply by the influence of pollination, the mother-plant 



