FERTILIZATION. 637 



for self-fertilization, inasmuch as the flowers either do not expand or their 

 buds are inconspicuous, and thus offer no attractions to insects. Such 

 flowers are very common on Violets, though commonly overlooked. They 

 yield more numerous seeds than the brighter-coloured flowers. 



Monoecious plants like the Hazel, which have the sexes in different 

 flowers on the same plant, may be subdivided into two classes, according 

 as the anthers are ripe before the pistil, or vice versa, the object clearly 

 being to favour the cross-fertilization of different plants. Many herma- 

 phrodite (structurally) flowers are organized in a similar manner. 



Dioecious plants, or those with the flowers of the two sexes on different 

 plants, must necessarily be cross fertilized, and, as we have just seen, many 

 plants structurally monoecious are rendered practically dioecious by the 

 different times at which the stamens and pistils respectively come to ma- 

 turity. Among dioecious plants the difference between the sexes is some- 

 times remarkably great : thus among Restiacese, sedge-like weeds of Aus- 

 tralia and the Cape, it sometimes happens that the male and female plants 

 of the same species are so different that the female much more closely 

 resembles the male of a totally different genus than it does the male of 

 its own species. Of course this applies only to the general habit and ap- 

 pearance of the stem and foliage, and not to the intimate structure of the 

 flowers. Lastly, we come to polygamous flowers, which Mr. Darwin 

 divides into two subgroups, according as the three sexual forms are 

 found on the same individual or on distinct plants. Of the latter case the 

 Ash is an example ; some trees bear in some seasons male flowers only, 

 others female flowers only, and others hermaphrodite blossoms. The Ash, 

 then, may be classed as tricecious. On the other hand the common Maple 

 bears all three sorts of flowers on the same tree, and is thus monceciously 

 polygamous. Other polygamous plants may be grouped into gyno-her- 

 maphrodites, inasmuch as they exist under two forms one of which bears 

 female flowers only, the other hermaphrodite flowers, as in the common 

 Thyme. Some of the Chenopodiums bear on the same plant hermaphro- 

 dite and female flowers, and may therefore be called gy no-monoecious. On 

 the other hand there are " andro-moncecious " plants, or plants bearing on 

 the same individual male flowers and hermaphrodite flowers, in some 

 species of Galium. No case seems to be known of andro-dicedous plants, 

 or plants producing hermaphrodite flowers on one individual, and males 

 on another. 



In the case of the sexually dimorphic flowers the pollen-cells are some- 

 times different in size in the long and short stamens respectively. At 

 other times no such difference can be detected, and, as a rule, there is no 

 relation between the size of the pollen and the length of the style. 



Pleterogonous flowers are still the exception. Mr. Darwin points out 

 that flowers already adapted by their structure for cross-fertilization 

 by insect agency do not need to become heterogonous ; hence, so far as is 

 known, there are no heterogonous flowers in Orchids, Labiates, Legumi- 

 nosse, and other large orders, the conformation of whose flowers is adap- 

 ted to insect agency. 



Though cross-fertilization is thus shown to be advantageous and very 

 general, yet there are some cases where every adaptation seems to be 

 made with the view of securing self-fertilization, as in the cleistogamic 

 flowers, above referred to. In Dombeya the staminodes or sterile sta- 

 mens are longer than the fertile ones, and are endowed with a power 



