200 
THALLOPHYTES. 
considerably exceeding the zygospore itself in size. In the contents of this sphere {F) 
may be recognised two chlorophyll - masses surrounded by oily protoplasm, which 
might have been distinguished even before their escape from the external layer of the 
zygospore. The contents now contract and become surrounded by a new wall {F) 
from which the older wall detaches itself as a delicate vesicle. After some time the 
protoplasm becomes constricted by a circular furrow, and splits into two hemispheres, 
each of which contains one of the two chlorophyll-masses {VI). Each hemisphere 
remains for a time naked, and again constricts itself; but this time the constriction 
does not advance to the centre; the hemisphere changes its form in other respects 
also, and each assumes the form of a symmetrically divided Cosmarium-ceW {VII), which 
surrounds itself with a wall of its own. The planes of constriction of the two cells 
derived from the zygospore cut the dividing plane of the zygospore itself at right angles ; 
. they themselves also lie crossed in the mother-cell at right angles to one another. The 
contents in each now arrange themselves in the manner above described ; the mother- 
cell-wall is absorbed and the new cells separate from one another. All these processes 
of germination are completed in one or two days. The new cells, whose outer wall is 
smooth, now divide in the usual manner, but the newly-grown halves increase in size 
and become rough on the outside {Fill — X) ; the two daughter-cells of each of the 
two cells produced from the zygospore have dissimilar halves, and the four cells pro- 
, duced by their further bipartition are therefore of two different forms : two have their 
halves equal and two unequal; the latter constantly produce by division one with 
equal and one with unequal halves. 
(b) The DiATOMACE^ ^ (Bacillarieae), a group extremely rich in species, follow natu- 
rally after the Desmidieae ; in particular they are allied to the Conjugatae by processes 
of development which correspond to the conjugation of the latter, or at least bear 
a certain resemblance to it ^. They bear a special resemblance to the Desmidieae in the 
form of their cells, in the manner of division, and in the mode of completion of the 
daughter-cells. Like the Desmidieae, the similar cells of the Diatomaceae may be united 
into rows, or may live entirely isolated. The tendency of Diatoms to secrete a soft 
jelly in which they live socially is found also in Desmids, although less strongly dis- 
played. In the same manner the movements of Diatoms are not altogether dissimilar 
to those of Desmids, and even the silicification of the cell-wall, which is very strong 
in the former, is found, though to a smaller extent, in Closterium and other Desmids; 
and the fine sculpturing of the silicious shell also finds an analogue, although in a coarser 
form, in the cell-wall of some Desmids. The Diatoms are the only Algae, except the 
Conjugatae, in which chlorophyll occurs in plates and bands, but in some forms it 
is also found in granules, and the green colouring matter is concealed, as in the 
chlorophyll-granules of the Fucaceae, by a buff'-coloured substance, Diatomin or Phy- 
coxanthin^. One of the most prominent peculiarities of Diatoms consists in their 
silicified cell-wall being composed of two separated halves or valves of unequal age, of 
which the older one partially envelopes the younger like the lid of a box. When 
the cell begins to divide, the valves separate from one another, and after the division 
of the contents into two daughter-cells, each of them forms a new layer at the plane 
of division which is adjusted by its turned-in margin (the girdle) to the girdle of the 
^ Lüders, Ueber Organisation, Theilung und Copulation der Diatomeen, Bot. Zeitg, 1862. — 
Millardet and Kraus discuss their colouring-matter in Compt. rend. vol. LXVI. p. 505 — Askenasy 
in Bot. Zeit. 1869, p. 799. — Pfitzer, in Hanstein's Bot. Abhandl., Heft II, 187 1, gives the most im- 
portant contribution to our knowledge of the Bacillarieae. [Quart. Journ. Micr. Sc., 1872, 1873.] — 
Borscow, die Sliswasser-bacillariaceen Russlands, 1873. 
^ [Thvi'aites first discovered the conjugation of the Diatomacese, Ann. Nat. Hist. 1847, 
vol. XX; see also Carter, Ann. and Mag. Nat. Hist. 1856. — Schmitz, Quart. Journ. Micr. Sc., 
1873, p. 14.5. — Smith, Synopsis of British Diatomacese.] 
^ \lPinmilaria viridis and Synedia splendens are green. Navicula fusiformis var. ostrearia is cobalt 
blue and communicates a green colour to the oysters which feed on it ; Nature, vol. XVI. p. 397.] 
