PHANEROGAMIA. 411 



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 stigmas and sta- 

 mens 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 proto- 

 gynous, 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 Composite, 

 Lobeliacese, and Campanulacese may be mentioned ; and of proto- 

 gynous flowers, Aristolochia, Arum, Scrophularia nodosa, and 

 some species of Plantago, but this condition 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 

 of honey, must first be mentioned. The peculiar marking of the 

 flower serves in many cases the purpose of guiding insects to the 

 nectary. The form of the flower, the situation of the honey, the 

 position of the stamens, and their relation to the other parts of the 

 flower, particularly to the stigma, the relative development in 

 point of time of the different parts, all these circumstances com- 

 bine and co-operate to secure cross-pollination, and sometimes to 

 aljow of the visits of particular insects only, as, for instance, of 

 butterflies with long probosces : though there are also cases in 

 which the insects must occasionally convey the pollen to the 

 stigma of the same flower. A simple arrangement of this kind 

 known as heterostylism or dimorphism, and which occurs in species 

 of Primula, Pulmonaria, Linum, Polygonum, etc., may be men- 

 tioned here. These plants have two forms of flowers ; in one 

 form the stamens are short and the style much longer, so that the 

 stigma projects above the anthers ; in the other form, on the 

 contrary, the anthers are on long filaments above the stigma ; 

 they are both so constructed that the anthers of one form stand on 

 the same level as the stigma of the other. From the position of 

 the nectary, and the form of the rest of the flower, an insect 



