THE ANNALS 
AND 
MAGAZINE OF NATURAL HISTORY. 
[SECOND SERIES.] 
No. 3. MARCH 1848. 
XVI. — Further Observations on the Diatomacese ; with descriptions 
of new genera and species. By G. H. K. Thwaites, Lecturer 
on Botany and Vegetable Physiology at the Bristol Medical 
School. 
[With two Plates.] 
Agreeably with the promise made in my last communication, I 
proceed to offer a few observations upon the facts there brought 
forward with reference to conjugation in the Diatomacece, and 
especially as to the bearing which these facts have upon the 
subject of impregnation in the higher tribes of plants. 
It may be desirable perhaps, by way of preliminary, to give a 
short general account of the phfenomena, as far as they have been 
observed, which present themselves in the course of development 
of a species of the Diatomacece. 
The frustules of a Diatomaceous plant, which are usually, as 
is well known, of very definite and often very beautiful figure, 
are continually undergoing fissij)arous division — that is, the con- 
tained endochrome of each one of these frustules divides into two 
portions, each of which developes around itself a cell- wall pos- 
sessing a form and character precisely similar to those of the 
original one. The process of fissiparous division continuing, ne- 
cessarily in course of time causes a very considerable increase in 
the number of frustules. There appears however to be a limit 
to this mode of propagation of the frustules, except by the inter- 
vention of another phsenomenon — namely conjugation, or a mix- 
ture of endochromes ; after which process tissiparoiis division 
proceeds as before. 
It seems probable that physiologically we ought to consider the 
numerous frustules which have originated from the primordial 
frustule (the sporangium or product of conjugation) not as so 
many individuals of a species, but rather as parts of one indi- 
vidual, which instead of, as takes place in the higher plants, 
cohering to form one structure, assuming forms more or less 
modified, and exhibiting a greater or less specialization of fimc- 
Ann. ^ Mag. N. Hist. Scr. 2. Vol. \. 11 
