4, 



454 PART III. THE CLASSIFICATION OF PLANTS. 



in the case of flowers which are conspicuous by their size, colour, 

 perfume, or by their secretion of honey, the conveyance is effected 

 by the insects which are attracted to visit the flowers ; such 

 flowers are said to be entomophilous. 



In those ambisporangiate flowers to which cross-fertilisation is 

 indispensable, or at least important, the most various contrivances 

 are exhibited for the purpose of hindering or limiting self-pollina- 

 tion on the one hand, and, on the other, of facilitating cross-pol- 

 lination ; or finally, in default of cross- pollination, of ensuring 

 ultimate self-pollination, this last, of course, only in those cases in 

 which the pollen of the flower itself is capable of fertilising it ; 

 for it is evident that self-pollination, even if not very advan- 

 tageous, is at any rate of some use to the plant. 



Among the contrivances for the prevention of self-pollination, 

 one of the simplest is the arrangement of the anthers and stigma 

 in such positions that the pollen cannot possibly reach the stigma 

 of the same flower, e.g. Aristolochia (Fig. 293) : or secondly, the 

 abortion of all the microsporangia in some flowers and of all the 

 macrosporangia in others ; in such flowers the organs in question 

 are present, but they are not functional. This is an approach to 

 the diclinous condition ; it occurs in the Tiger- Lily, in which the 

 anthers are commonly abortive in some flowers and the ovaries in 

 others. Thirdly, dichogamy frequently occurs, that is, that the 

 stigmata and stamens attain their functional activity at different 

 times : flowers in which this occurs are either protandrous, that is, 

 the anthers are first developed and have already shed their pollen 

 when the stigma of the same flower is capable of receiving it; or 

 they are protogynous, that is, the stigma is fully developed before 

 the anthers of the same flower are ready to shed their pollen : in 

 the latter case self-pollination is obviously only excluded if the 

 stigma is withered before the pollen is shed ; there are, however, 

 protogynous flowers in which the stigma remains fresh for a long 

 time and which may be pollinated by their own pollen. As 

 examples of protandrous flowers, those of the Umbelliferse, and 

 most of the Compositae, Lobeliacese, and Campanulaceae may be 

 mentioned ; and of protogynous flowers, Aristolochia, Arum, 

 ticrophularia nodova, and some species of Plantago, but this con- 

 dition is less common than the preceding. 



Among the contrivances which lead to the cross-pollination of 

 flowers by the agency of insects, the means of tempting insects to 

 visit the flowers, such as bright colours, odours, and the secretion 



