ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 449 



phycin can be demonstrated in many species of Nostoc. This intercom- 

 munication between the cells is always interrupted on the formation of 

 heterocysts. During the transformation of ordinary cells into heterocysts, 

 the walls become thicker, the cellulose-like substance collecting especially 

 round the pores through which the strands pass, and eventually altogether 

 closing them. The cells of the hormogones present the same peculiarity of 

 structure. 



Cells which are about to change into spores cease dividing transversely, 

 and increase slightly in size ; the contents become slightly darker in colour, 

 and the gelification of the outer layers of the cell-wall ceases. The spore 

 is the result of a true process of rejuvenescence, its cell-wall being formed, 

 not from that of the old cell, but out of its contents. The mature spore 

 possesses a distinct outer layer or exospore. The spores thus formed are 

 true examples of cystidia. 



Intercellular communication can be detected also in the Scytonemaceee, 

 Stigonemacese, and Eivulariacese, which present no difference, in other 

 points of j)hysiological importance, from the Nostocace^. 



The OscillariacesB are conveniently divided into two groups, according 

 as their movement is straight or spiral. Of the latter class Oscillaria may 

 be regarded as the type. Contrary to the general statement, the author 

 finds the filaments to be always invested in a delicate gelatinous sheath, 

 which can be readily seen on treatment by alcohol. There is no sharp 

 differentiation between the cell-wall and protoplasmic contents, the former 

 being but a slightly differentiated peripheral portion of the latter. Lynghya 

 and Microcoleus present the same structure as Oscillaria. The helicoid 

 motion of the filaments of Oscillaria is due to the axis of the filaments never 

 being perfectly straight ; the form and direction of the apical portion 

 always differ from that of the main portion of the filament ; and this apical 

 portion is always protected by a kind of cap. In the case of those species 

 which live associated in dense tufts, the motion is incessantly interrupted 

 and altered in a variety of ways. The direction of the motion is strongly 

 affected by light ; but the author states that it is an error to suppose that 

 this motion is constant at all periods of development of the filaments. It 

 is confined to the reproductive period, when the colony is multiplying and 

 extending its geographical area by the production of hormogones. 



Heterocystous Nostocacese.* — Under this name, MM. E. Bornet and 

 C. Flahault include all the Phycochromacese (Cyanophycefe) which are re- 

 produced by hormogones, and in which the cells are of two distinct kinds, 

 ordinary vegetative cells; — and special cells, terminal hair-like cells or 

 heterocysts, i. e. the Eivulariaceae, Sirosiphonacese, Scytonemaceae, and 

 Nostocese. The structure of these families is described in reference to the 

 cells, the trichomes, the sheath, the heterocysts, the branching, the hormo- 

 gones, and the spores. 



The authors have been unable to detect the presence of a distinct 

 nucleus in any one species. The cells attain their highest degree of 

 differentiation in the Eivulariacese. They propose to limit the term 

 " trichome " to the row of cells or masses of protoplasm, " filament " to the 

 trichome with its gelatinous envelope. The gelatinous sheath, when thick, 

 often consists of a system of lamellsB, crossed by transverse lines or folds, 

 resulting from the extensibility of the integument of the cell, and the 

 different capacities for gelification of its layers. Heterocysts occur in all 

 the forms except a few species of EivulariacesB ; they may be larger or 

 smaller than the vegetative cells. In certain Eivulariacese (JRivularia, Galo- 



* Ann. Sci. Nat. (Bot.), iii. (1886) pp. 323-81 ; iv. (1886) pp. 343-73. 



