446 



VEGETABLE PHYSIOLOGY 



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 bores 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, con- 

 tains a single antheridium, which produces two gametes. 



FIG. 182. DEVELOPMENT OF THE ARCHEGONITJM OF THE FERN. (After Kny.) 



1, 2, 4, 5, 7, 8, 9 Successive stages ; 3, 6 transverse sections of the 

 neck region of 4 and 5. 



These are generally undifferentiated portions of protoplasm, 

 but in Ginkgo, Zamia, and in some species of Cycas they have 

 been found to be ciliated antherozoids. In the Angiosperms 

 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 antherozoi'd, with the 

 oosphere, becomes possible by a deliquescence of the sepa- 

 rating 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 



