26o THALLOPHVTES. 



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 {V) 

 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 {FI). 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 {FII), 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 Gonjugatae 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 

 Gonjugatae, 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 



* Luders, 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 11, 1871, gives the most im- 

 portant contribution to our knowledge of the Bacillarieae. [Quart. Journ. Micr. Sc, 1872, 1873.] — 

 Borscow, die Siiswasser-bacillariaceen Russlands, 1873. 



^ [Thwaites 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] 



3 [Pinnularia 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.] 



