436 VEGETABLE PHYSIOLOGY 



effected by the bringing together of the two spores con- 

 cerned in developing them. The microspore or pollen 

 grain is carried by various means to the neighbourhood of 

 the megasporangium ; in the Gymnosperms it falls upon 

 the megasporangium itself ; in the Angiosperms upon the 

 stigma of the pistil in which the megasporangia are hidden. 

 When it germinates the prothallium or gametophyte takes 

 the form of a long tube, which makes its way through the 

 intervening tissues till it reaches the megaspore itself, 

 close to the archegonium in the first case, and to the oosphere 

 in the Angiosperms, where there is no archegonium. In 

 the Gymnosperms the tube, the so-called pollen tube, 

 contains a single antheridium, which produces two gametes, 

 which are generally undifferentiated portions of protoplasm, 

 but which in Ginkgo and in some species of Cycas have 

 been found to be ciliated antherozoids. In the Angio- 

 sperms there is no antheridium, but two gametes are pro- 

 duced, which show no differentiation. From the great 

 preponderance of the nuclear matter they contain they are 

 often spoken of as the generative nuclei. 



Fusion of the latter, or of the antherozoid, with the 

 oosphere, becomes possible by a deliquescence of the 

 separating walls, and in all cases a single male gamete fuses 

 with an oosphere. Where several oospheres are found 

 upon the same prothallium, as in the Gymnosperms, more 

 than one may be fertilised by gametes from the same 

 pollen-tube. This occurs in certain of the Cupressinece ; it 

 is rendered possible by a multiplication of the male gametes, 

 which takes place by ordinary processes of division ex- 

 hibited by them as they pass down the tube. Several 

 embryos may thus arise in the seed. Usually however 

 only one of these undergoes a normal development. 



In certain cases in the Angiosperms the second of the 

 generative nuclei has been observed to fuse with the two 

 polar nuclei or the definitive nucleus of the embryo-sac. 

 The extent to which this takes place has not yet been 

 determined and its interpretation is not at present easy. 



