14 INTRODUCTION 
(a2) 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. 
(6) 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 flowers.) 
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 
Jrom another flower surpasses in fertilizing effect pollen belonging to the same 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 she development of the arrangements 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. 1, 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. 
1 ‘Ulteriori osservazioni sulla dicogamia nel regno vegetale.’ 
2 Professor in Naples, and till 1893 ia Bologna, 
