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. 
Tig. 182.—DrvELopMENT OF THE ARCHEGONIUM 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 nuclet. 
Fusion of the latter, or of the antherozoid, 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 oogpheres are found 
upon the same prothallium, as in the Gymnosperms, more 
