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 antheridium, 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 nuctlet. 
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 Cupressineg; 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. 
