630 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
of Mexico. They belong to the genera TriploporeUa , Linoporella g. n., 
and Neomens, and are frequently in a very good state of preservation. 
From the fact that, with the exception of TriploporeUa , all the pre- 
tertiary Dasycladaceae belong to the Dasycladus- type, the author derives 
the conclusion that this is the older type, from which the Acetabularia- 
type is descended. 
Unicellular Colonial Algae.* — In an important contribution to our 
knowledge of colony-forming algae, Herr G. Senn treats of the genera 
Coelastrum , Scenedesmus, Dictyosphserium , and Oocardium , from both a 
structural and a physiological point of view. 
Coelastrum. This genus is described as being composed of spherical 
or polygonal cells, with a chlorophyll-green bell-shaped chromatophore, 
a pyrenoid, and a starch-envelope. In the centre is the nucleus con- 
taining a single nucleole. Reticulate strings of protoplasm lie on the 
cell-wall, isolated strings crossing the cell-cavity. Multiplication takes 
place by repeated bipartition into immotile daughter-cells, until the 
number amounts to thiity-two. The membrane of the mother-cell then 
bursts into two halves which still remain connected, compound coenobes 
being thus formed. When the supply of oxygen is deficient, spherical 
coenobes are formed, the cells being united together by a mucilaginous 
layer. With deficient nutriment the cells pass into a resting condition, 
characterised by the formation of a reddish-yellow oil. As regards its 
systematic position, the author regards Coelastrum as a typical member 
of tlie Pleurccoccaceae, rather than as nearly related to Pediastrum and 
the Hydrodictyaceae. The genus consists of six species, the diagnoses 
of which, with their rather extensive synonymy, are given, and the 
stiucture and life-history of three described in detail. 
In Dictyosphserium pulchellum no cellulose could be detected in the 
cell-wall. The jelly is not an amorphous pioduct of the swelling of 
the mother-cell, but a distinct substance of evident rod-structure ex- 
creted from the cell, inducing the arrangement of the cells in colonies 
of definite form. No formation of resting cells or of swarm-spores 
could be established. 
Oocardium. This genus belongs not to the Tetrasporeae, but to 
the Desmidiaceae, and must be placed near Cosmocladium and Cosmarium. 
The cells are divided into two halves by a shallow constriction. The 
valve-side is obovate ; the girdle-side has an obcordate appearance on 
its broader side ; on the narrower side it presents the appearance of a 
slightly constricted Cosmarium. It forms calcareous incrustations, 
witlnn which the amorphous jelly takes the form of stalks. 
The formation of colonies may take place in these algae in five 
ways : — (1) by adhesion ; (2) through union by remains of the mother- 
cefl-wall ; (3) by the excretion of jelly ; (4) by the association of free 
swarm-cells ; (5) by protoplasmic connections from cell to cell. 
While not going so far as Chodat in his view of the polymorphism 
of algae, the author asserts that all the three species of Ccelastrum 
examined by him may originate from single cells which are indis- 
tinguishable from the spherical Protococcoideae. 
* Bot. Ztg., lvii. (1899) l te Abt ; , pp. 39-104 (2 pis. and 39 figs.). 
