366 MR. G. S. WEST ON 
On Variation in the Desmidiex, agd its Bearings on their 
Classification. By G. S. Wesi, B.A., A.R.C.S., Scholar 
of St. John's College, Cambridge. (Communicated by 
W. Wzsr, F.L.S.) 
[Read 4th May, 1899.) 
(PrATEs 8-11.) 
Page 
I. Introduction... .....Le eese eene 366 
Il. Variations in Form and Symmetry | ..................... 376 
111, Variations of Cell. eontents, eaaa enere 399 
IV. Variations in Conjugation ........... esce 402 
V. Some Interrelationships of the  Desmidies as 
deduced from a study of their Variation............ 403 
I.— Introduction. 
Tus family of minute plants, so remarkable for beauty and 
extraordinary variety of form, is characterized by the constancy 
of those features and combinations of features which specifically 
distinguish the- different members of the group. These cha- 
racters are found to be present in specimens of the same species 
obtained from divers parts of the world, and, although often in 
themselves slight, are as easily discernible to the practised eye 
of the observer as are the specific characters of higher plants. 
On the examination of a large number of specimens of one 
species from many widely separated localities, certain examples 
are sure to be found which exhibit some variation from the 
typical plant, and without a very careful study of the species it 
is dif&eult to determine whether this variation is merely of a 
transitory or accidental nature, the specimen being the direct 
offspring of some type-form, or whether it constitutes a true 
variety produced by a gradual evolution from the original type. 
The ordinary method of increase in this family of unicellular 
plants * is by the division of the mother-cell into two exactly 
similar daughter-cells, each constituted of one of the half-cells 
* The Desmidiez may be regarded as a family of unicellular plants evolved 
by retrogression from some sexually differentiated ancestors (cfr. West & 
G. S. West, ‘Obs. on Conjugate,’ Ann. Bot. vol. xii. March 1898). Some few 
of the genera are filamentous, but these embrace only a very small minority of 
the known species, and they, as a rule, easily dissociate into individual cells, 
