9 6 
FLORAL MECHANISM ETC. 
A widespread phenomenon in hermaphrodite flowers is 
dichogamy or sex- separation in time ; the pollen is ripe 
before the stigma, protandry , or the reverse, protogyny. A 
flower in which stamens and stigmas ripen together is termed 
homogamous , but true homogamy is rare. If the dichogamy 
be complete, i.e. if the first stage be over before the second 
begins, the result will be the complete prevention of self- 
pollination, while at the same time the anthers and stigmas 
may occupy positions as close as possible to one another, or 
even, by aid of movement, occupy the same position, at 
different times. Completely dichogamous flowers are rare, 
and are usually such as are so attractive to insects as to 
ensure being sufficiently visited for cross-pollination, eg. 
Aeschynanthus, Canna, Clerodendron sp., and some wind- 
pollinated flowers, eg. Parietaria. Nearly all flowers are 
more or less dichogamous, but there is generally a consider- 
able period of overlap of the male and female stages, during 
which self-pollination is frequently possible. This is more 
or less guarded against by numerous mechanical arrange- 
ments, to be dealt with below. Dichogamy is a variable 
phenomenon, the same species showing different degrees of 
it in different localities, and at different periods of the year. 
Protandry is most common in insect-visited flowers (good 
examples of simple protandry are found in Umbelliferae 
and Caryophyllaceae), protogyny in wind-pollinated flowers ; 
the latter also occurs in a considerable number of entomo- 
philous forms, eg. many Araceae, Paris, Colchicum, Asarum, 
Chimonanthus, Amorpha, Goethea, Aesculus, &c. 
Simple dichogamy 7oith movement of the essential organs 
so as to place first one and then the other in the track of an 
entering insect, is very common. Extreme cases occur in 
Aeschynanthus, some Clerodendrons, &c. Others, in which 
there is more or less possibility of autogamy, may be seen 
in Scabiosa, Lonicera, Scrophularia, Teucrium, Umbelliferae, 
Caryophyllaceae, Delphinium, Aconitum, Geranium, Ruta, 
Epilobium, Malva, and hundreds more. 
Passing on to the more purely mechanical arrangements 
for regulating pollination, we find in the Orchids (q.v.) one 
which in nearly all cases renders the flowers incapable of 
pollinating themselves. These plants are perennials with 
much vegetative reproduction ; this obviates the risk of 
