14 INTRODUCTION 



(a) Homoclinous pollination is prevented : 



1. By dicliny: for each pollination two visits of an insect are necessary. 



2. By dichogamy: for each pollination two visits of an insect are necessary. 



3. By herkogamy : for each pollination a single visit of an insect is enough, 



inasmuch as in visiting such flowers the insect brings pollen to the 

 stigma, and simultaneously carries away pollen to another flower. 

 (3) Homoclinous pollination is not hindered. For each pollination a single 

 visit of an insect suffices. 



1. Heterostyly. 



2. Homostyly. 



B. Flowers that are pollinated without the help of an external agent. (Cleisto- 

 gamous flcwers.) 



Loew (op. cit., p. 153) gives the conclusions of Axell as follows : Phanerogams 

 are normally provided with open (chasmogamous) flowers, which can accordingly be 

 crossed with those of other individuals. The active agents in the process are wind 

 and insects. When the possibility of fertilization with foreign pollen is excluded, the 

 phanerogams have flowers which do not open (cleistogamous flowers). They then 

 fertilize themselves within the closed floral envelope. Among chasmogamous flowers 

 homoclinous pollination is sometimes impossible (dicliny), sometimes prevented 

 (dicho- and herkogamy), sometimes impeded (heterostyly), sometimes not impeded 

 (homostyly) : heteroclinous pollination is possible in equal degree in all. Pollen 

 from ajiother flower surpasses i7i fertilizing effect polleti belonging to the sarne flower, 

 and crossing is accordingly the common mode of sexual reproduction in all cases. 

 Fertilization with foreign pollen is also more advantageous than fertilization with 

 pollen of the same plant. Plants whose sexual reproduction is better assured and in 

 which it is effected with a greater economy of material, space, and time, are regarded 

 by us as higher in respect to sexual propagation. The security of sexual propagation 

 is greater as we pass from wind-pollinated to insect-pollinated plants. In anemophilous 

 plants we find gradations from dioecism to monoecism and to protogyny : in ento- 

 mophilous plants from dioecism to monoecism, protandry, herkogamy, heterostyly, 

 and homostyly. The economy of material, space, and time increases in the same 

 order. We see therefore that the developmejit of the arrangemetits for sexual union 

 among the phanerogams has been and still is in this direction. 



In his work, ' Later observations on Dichogamy in the vegetable kingdom ' * 

 (Part I, 1868, 1869; Part II, fasc. i, 1870; fasc. 2, 1875. Atti Soc. ital. sc. nat., 

 Milano, xi, xii, and xvi), Federico Delpino ^ endeavoured with much success to arrange 

 the previously known types of floral arrangement into oecological groups, and to set 

 forth the relationships, so far as regards flower pollination, of kindred families. The 

 classification of adaptations for pollination throughout the whole plant kingdom set 

 forth by Delpino is still of value in some respects. It is therefore given in the 

 second section of this volume. 



Delpino was perhaps less fortunate in his attempt to refer all floral forms to 

 a number of types, of which he distinguished 47, grouped into 13 classes. 



^ ' Ulteriori osservazioni sulla dicogamia nel regno vegetale.' 

 - Professor in Naples, and till 1893 in Bologna. 



