134 
Agnes Robert son. 
CYTOLOGY AND CLASSIFICATION. 
A Lecture 
By Agnes Robertson, D.Sc. 
T seems at first sight as though the intimate way in which the 
structure of the living cell is bound up with the physiological 
functions it has to perform would prevent cytology throwing much 
light on classification. The cell may be compared to a busy 
modern town, with an energetic and progressive municipality, bent 
on having thoroughly up-to-date sanitation, water-supply, locomotion, 
etc., and consequently making a clean sweep of any antiquated or 
obstructive buildings. The inevitable result of this is of course the 
removal of many historic land-marks. But just as in the most 
modernised town the antiquary generally manages to find some 
corner which the besom of reform has failed to reach, so the 
morphologist may hope for some clues to the pedigrees of the 
plants with which he is concerned from a consideration of the less 
obvious points in their cytology. 
In the lower comparatively unspecialised plant types, in which 
the whole body is made up of similar cells, ( e.g . certain Algae), 
almost the only place in which we can look for characters of 
taxonomic value is in the structure of the cells themselves. If we 
compare the cells of two such plants living under the same 
conditions, it seems safe to assume that the characters in which 
they differ are phyletic, for such differences cannot be accounted 
for as a modern response to external conditions, (compare for 
instance Spirogyra with its spiral, and Zygnemn with its stellate 
chromatophores). The importance of cytological features in 
classification holds good throughout the Algae. The unicellular 
genus Chlamydomonas contains nearly thirty species, discriminated 
by their remarkably constant cytological characters ! We may 
imagine that the Algae are still trying experiments as to the most 
efficient kind of cell and chromatophore, and have not yet reached 
a definite conclusion. The Siphoneae are testing the advantages of 
a coenocytic body, but have “ only succeeded in producing the 
elaborate but puny mockery ” of the higher plants which we find in 
Caulerpa. Another group, the Cladophoraceae, has adopted an 
incompletely septate body, and so on. When we turn from these 
smaller groups to the half-dozen or so large classes into which 
the Algae are divided, we find that they are based almost exclusively 
on cytological characters. For instance, the class Heterokonteae, 
