REPRODUCTION 437 



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 anther idium, which produces two gametes. 

 These are generally undifferentiated portions of protoplasm, 

 but in Ginkgo and in some species of Cycas they have 

 been found to be ciliated antherozoids. In the Angio- 

 sperms there is no antheridium, but two gametes which 

 show no differentiation are produced in the pollen tube. 

 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 many families of 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. 

 Some observers hold that the fusion of the cells has 

 nothing sexual about it, but is nutritive only ; others look 

 upon the so-called endosperm which results as an abortive 

 second embryo. 



