ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 687 



mode of reproduction. Then follows a guide to their study, the 

 mode of collection and cleaning, the best instruments to employ in 

 their examination, the most advantageous methods of preparation and 

 media for mounting. A glossary is given of all the terms employed 

 in diatomography, and an account of the best systems of classification, 

 the one followed in the book being that of Prof. H. L. Smith, de- 

 pendent entirely on the characters of the siliceous envelope, without 

 reference to the endochrome. In the determination of species, since 

 the work is intended for beginners as a guide to the naming of 

 diatoms, the term (" species ") is used in its widest significance ; a 

 comparatively small number of primitive types being adopted under 

 which the secondary types are ranged. A bibliography is appended ; 

 followed by the detailed descriptions of the genera and species. 



Lichenes. 



Structure of Lichens.* — Herr H. Zukal describes the " gono- 

 cysts" of Manzonia Cantiana^ where they occur on the surface of the 

 thallus, and especially on its outer margin. On the blue-green short- 

 celled hyphae are found globular " capsules " of various sizes, opaque, 

 of a dark colour, and consisting of one, two, four, or more chambers, 

 each containing a green spherical or elliptical cell, or gonidium. 

 Sooner or later the wall of this capsule becomes mucilaginous, and 

 the gonidia are now inclosed by hyphag from the adjacent thallus, the 

 thallus itself thus increasing in size. In other cases the gonocysta 

 become detached from the thallus, are carried away, and, when 

 reaching a suitable nidus, develope into a new thallus formed from 

 the fragments of hyphae which remain attached to these. The gono- 

 cysts are now formed by gonidia making their appearance on the 

 margin or surface of the thallus, which become enveloped in a thick 

 dark-coloured membrane. The gonidium divides within this "cap- 

 sule " into a number of daughter-cells, until the wall of the capsule 

 finally becomes absorbed. 



The gonangia are roundish bodies, consisting of a brown pseudo- 

 parenchymatous envelope, in connection with a hypha, and containing 

 in their cavity a number of green pleurococcus-liko cells, which are 

 not formed cither in the envelope or from it. They occur in large 

 quantities in all situations, especially on bark and wood, and are 

 surrounded by the hypha), which in the lower cortical layers are 

 colourless and thin-walled, on the surface thicker, brown, and com- 

 posed of short colls, forming the pseudo-parcnchymatous envelope 

 round the gonidia or algal cells. The gonangia apparently assist in 

 the dinsomination of the lichens, and are found in those species which 

 inhabit bark, though comparatively rarely. 



Many lichens pass, when under certain conditions, into a vegeta- 

 tive condition cliaracterized by peculiar changes in the contents of 

 the hyphal cells. The protoplasm becomes nearly homogeneous, 

 strongly rcfringont, and lias a distinct green tint. It then breaks up 

 readily into regular minute sphericul bodies, uniform in size, the 



* Denkuchr. K. Akad. Wm Witii, xlviii. See Hcdwigia, xxiv. (1885) p. IS. 



