632 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of their leaves. He describes and figures various stages in the life- 

 history of the Chlorochytrium and states that the fission-products of the 

 alga are often associated with diatoms. These diatoms appear to be 

 almost always immature, and the author can only account for then- 

 presence inside the Lemna by one of two hypotheses. Either («) the 

 diatoms have, like the algas, entered through the stomata ; or (Z>) they 

 have been produced in situ by a transformation of the fission-products 

 of the alga. A certain number of reasons are adduced to show that the 

 latter of the two hypotheses is in the author's opinion the correct one. 



Structure and Division of Diatoms.* — C. Mereschkowsky pub- 

 lishes a preliminary report of his latest researches on this subject. He 

 gives details of the structure and manner of division of Pinmdaria, 

 Navicula, Gyrosigma, Stauroneis,- Neiclium, Achnanthidium, Cymbella, 

 a new genus Placoneis, Sellaphora, Microneis, Eunotia, Nitzschia, 

 Hantzschia, Gampylodiscus, Surirella, Stenopterobia (of which the author 

 describes a new species), Amphiprora, Stauronella, Tabellaria, and 

 Fragilaria. Figures are given showing the division in most of these 

 genera. The author concludes his paper by a note on the " law of 

 alternation of the plane of division." He finds that along the line of 

 evolution the plane of division changes regularly and alternately from 

 longitudinal to transverse, from group to group. Thus, if one follows 

 the line of evolution of the Raphideae, the genus Libellus, which may 

 be considered the general ancestor of the Polyplacatge, divides longi- 

 tudinally. The next group, Diplacatse, represented by Navicula, &c, 

 divides transversely. Tetraplacatae, the following group, divides longi- 

 tudinally, and so on. The author does not attempt to explain this 

 interesting fact, but he suggests that there may be something in common 

 between the alternation in the division of the growing points of such 

 algae as Sphacelaria, C/mtopteris, Gladostephus, &c, and the alternation 

 of division in the successive groups of diatoms. In the one case the 

 time occupied to produce the alternation is limited to the formation of 

 successive cells in one and the same plant ; in the other case a change 

 of the plane of division demands an immense length of time, sufficient 

 for the evolution of a new group. 



Auxospores of Diatoms.t — C Mereschkowsky gives a full and 

 lucid account of the various types of auxospore and their mode of 

 formation. In doing so he disagrees on certain points with G. Karsten, 

 principally concerning the evolution of the types and their respective 

 order of development. As presented by Mereschkowsky, the auxospores 

 may be divided into two classes : (1) Asexual auxospores, formed without 

 copulation and from a single individual ; and (2) sexual auxospores, 

 formed by the copulation of two cells. These classes are found to 

 coincide with the division of diatoms into immobile and mobile, the 

 asexual auxospores being characteristic of immobile, the sexual auxospores 

 of mobile species. The fact that these two forms of classification on 

 different lines coincide, strengthens the present author in his views as to 

 the evolution of the various types. 



* Bull. Soc. Imp. Nat. Moscou, No. 1 (1903) pp. 149-72. 

 t Ann. Sci. Nat., xvii. (1903) pp. 225-62 (20 figs, in text). 



