GROUP IV. — PHANEROGAMIA. 453 



The bringing of the microspore into such close relation is what is 

 termed pollination. When the pollen of anyone flower is brought 

 into relation with the macrosporangium of the same flower, the 

 case is one of self-pollination ; when the pollen of any one flower is 

 brought into relation with the macrosporangium of another flower 

 (whether on the same plant, or on another plant of the same 

 species), the case is one of cross-pollination. 



The microspores when so brought are placed under conditions 

 of moisture and nutrition favourable to their germination. In 

 Gjmnosperms, where there is no ovary and no stigma, the micro- 

 spore is brought into direct contact with the micropyle of the 

 macrosporangium. In the Angiosperms, where there is an ovary 

 and a stigma, the microspores cannot come into direct contact 

 with the macrosporangium ; they fall upon the stigma and ger- 

 minate on its moist surface ; the pollen- tubes then grow down into 

 the ovary, down the style, if there is one, and finally enter the 

 ovules (see Fig. 280). 



In certain cases flowers, of course ambisporangiate, are so 

 modified as to ensure self-pollination: instances of this are afforded 

 by species of Viola, Lamium am^^lexicatile, 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 monosporangiate flowers (e.fj. 

 Gymnosperms) 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 ambisporangiate flowers, pollination is effected by the 

 transfer of pollen irom 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 3-et 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 flowera (e.g. 

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

 the agency of the wind, when they are said to be aneviophilous', but 



