GROUP IV. PHANEROGAMTA. 455 



of honey, must be first 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 

 allow 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 Tieterostylism 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 

 visiting it is obliged to take up the same position at each visit ; 

 consequently after it has visited a flower of the one form, when 

 it visits a flower of the other form, it touches the stigma of the 

 latter with the same part of its body with which in the first 

 flower it brushed the anthers, and thus the pollen which it 

 carried away with it from the anthers of the one flower is trans- 

 ferred to the stigma of the other. Observations made by arti- 

 ficially transporting the pollen have shown that fertilisation is 

 most complete when the pollen of stamens of a certain length is 

 conveyed to the stigma of a style of the same length. The same 

 is the case with trimorphic plants, e.g. Oxalis, Lythrum Salicaria : 

 in these, three forms of flowers occur with three different lengths 

 of styles and stamens. 



As examples of more complicated contrivances for the purpose 

 of securing cross- pollination, Aristolochia and Epipactis may be 

 described. 



The flower of Aristolochia Clematitis (Fig. 293) is protogynous ; 

 insects can penetrate without difficulty down the tube of _the 

 perianth, which is furnished on its internal surface with hairs 

 which point downwards, and they thus convey the pollen they have 

 brought with them from other flowers, to the stigma ; the hairs, 



