139 
Cytology and Classification. 
paternal characters, and that the cytoplasm, in the cases where 
it takes part in embryo formation, has a merely nutritive function. 
The Angiosperms are very sharply marked off from the 
Gymnosperms by the cytology of their reproductive organs. The 
endosperm and archgegonia of a gymnospermous ovule can be 
homologised without difficulty with the female prothallus of a fern 
or Lycopod, but at present the structures in the angiospermous 
embryo-sac stand alone, unless, as has been suggested, the gulf is 
to some extent bridged by such a form as Gnetum Gnemon, in which 
the archegonia are reduced to egg cells, and most of the endosperm 
tissue is developed after fertilisation. It is a question, however, 
whether this actually represents a stage in the evolution of the 
angiospermous ovule, or is merely a blindly ending side branch of 
the Gymnosperms, which has happened to develope in rather the 
same direction as the Angiosperms. In sub-dividing the huge group 
of the Angiosperms cytological evidence is of little avail. The 
extraordinary constancy of embryo-sac structure throughout the 
group, and the occurrence of “double fertilisation,” both in Dicoty 
ledons and Monocotyledons, point to the ancientness of the whole 
stock, but give no help in determining inter-relationships. However, 
“where differences exist between the Archichlamydeae and the 
Sympetalae, Monocotyledons are connected in these particulars 
with the Archichlamydeae 1 .” 
Pollen development furnishes one character of a certain 
slight taxonomic value, for it is found to be characteristic of Mono¬ 
cotyledons to produce four microspores in one plane, whereas in 
Dicotyledons they are arranged in a pyramid. Such a difference 
as this can hardly be explained except as an ancestral trait. 
Enough has perhaps been said to justify the conclusion that 
cytology, though it is such a new science, has already given some 
help in questions of classification and phylogeny. The fertilised 
egg of any plant, though but a single cell, bears that within itself 
which determines within narrow limits the whole course of the life- 
history, and the eggs of any two species must possess (if only we 
could observe and define them !) perfectly definite characters corre¬ 
lated with the specific differences exhibited by the mature plants. 
If ever perfected methods of observation should enable us to detect 
these subtle distinctions, Cytology will become the very basis of 
Classification. 
1 E. N. Thomas. “ A Consideration of the Bearing of Fertili¬ 
sation Phenomena and Embryo-sac Structure on the Origin 
of Monocotyledons.” Brit. Ass. Rep., Southport, 1903, p. 857, 
