5 6 
REPRODUCTIVE ORGANS 
The spermatozoids escape from the antheridium when 
ripe and swim to the archegonia (chemotropism or chemo- 
taxis ; p. 36) ; one of them unites with the ovum in an 
archegonium, the process being termed fertilisation. The 
resulting cell or zygote soon germinates and grows into a 
new leafy asexual plant. It is at first attached to the 
prothallus by an organ termed the foot, until it can feed 
itself. The life-history may be graphically represented thus 
(fern) : 
Leafy Plant or 
Sporophvte 
t 
Zygote 
Sporophylls Sporangia Spores 
jSpermatozoid- 
I Ovum -< 
-Antheridia) Prothallus or 
-Archegonia \ Gametophyte. 
It will be seen that there is an Alternation of Generations, 
sexual and asexual. This is a general feature of all the 
plants with which we have to deal, but in the Spermaphyta 
the gametophyte stage is much reduced and is no longer 
on the ground. The megaspore germinates in the ovule or 
megasporangium ; the microspore or pollen-grain is carried 
to its neighbourhood by wind, insects, or otherwise. This 
preliminary operation is called pollination ; the microspore 
germinates, and gives rise to a spermatozoid (Cycadaceae 
&c.) or to a tube ( pollen-tube ) which burrows to the ovum. 
For further details of these phenomena see articles 
Pteridophyta, Filicineae, Gymnospermae, Angiospermae, 
Chalazogamae, &c. 
Cross- and Self-Fertilisation. If, of the ova of a 
plant A, some be fertilised by male cells of A, some by 
male cells of a different plant of the same kind, B, will 
there be any difference between the offspring of self-fertilisa- 
tion (A x A) and that of cross-fertilisation (B x A)? The 
answer to this question is supplied by the experiments of 
Darwin, detailed in Cross- and Self Fertilisation of Plants. 
In each species seeds obtained by self-fertilisation (AA), 
and by fertilisation with pollen from a distinct plant (BA), 
were planted in pairs, one of each kind, in pots, and thus 
brought into competition. When fully grown the heights 
and weights were measured, and also the number and 
weight of seed produced. AA were again self-fertilised, BA 
