410 PART IV. CLASSIFICATION. 



brought into Direct contact with the macrosporangium (Fig. 241). In 

 the Angiosperms, where there is an ovary and a stigma, the micro- 

 spores cannot come into direct contact with the macrosporangium ; 

 they fall upon the stigma and germinate on its moist surface : the 

 pollen-tubes then grow into the ovary, down the style if there is 

 one, and finally enter the ovules. 



In certain cases flowers are so modified as to ensure self-pollina- 

 tion : instances of this are afforded by species of Viola, Lamium 

 amplexicaule, Oxalis Acetosella, and others, where the plant (in 

 addition to the ordinary flowers) bears inconspicuous flowers which 

 do not open, and in which self-fertilisation is perfectly effected by 

 the pollen ; these peculiar flowers are said to be cleistogamous. 



In the great majority of Phanerogams, however, cross-pollina- 

 tion is the rule. In the case of diclinous or dioacious plants (e.g. 

 Gymuosperms) it is clear that pollen must be conveyed from a 

 staminate to a carpellary flower. It is also known that in a great 

 number of monoclinous flowers, pollination is effected by the trans- 

 fer of pollen from one flower to another : in some of these cases it 

 has been demonstrated that it is only the pollen of another flower 

 which can effect fertilisation ; in other cases, that the pollen of the 

 same flower, though not absolutely useless, has less fertilising 

 power than that of another flower; and in yet other cases, that 

 though the pollen of the flower itself has sufficient fertilising 

 effect, yet the progeny is less vigorous than when pollen is 

 supplied from another flower. 



The conveyance of pollen from one flower to another is effected, 

 in the case of a number of plants with inconspicuous flowers (e.g. 

 Gymnosperms, Grasses, many Dicotyledonous Forest-trees), by the 

 agency of the wind, when they are said to be anemopliilous ; but 

 in the case of flowers which are conspicuous by their size, colour, 

 perfume, or by their secretion of honey, the conveyance is effected 

 by the insects which are attracted to visit the flowers; such 

 flowers are said to be entomophilous. 



Among the contrivances for the prevention of self-pollination 

 in monoclinous flowers, one of the simplest is the arrangement of 

 the anthers and stigma in such positions that the pollen cannot 

 possibly reach the stigma of the same flower, e.g. Aristolochia 

 (Fig. 243). Or secondly, the abortion of all the microsporangia in 

 some flowers and of all the macrosporangia in others ; in such 

 flowers the organs in question are present, but they are not 

 functional : this is an approach to the diclinous condition ; it 



