BlhUographical Notices. 129 



more space than can be allotted to the whole of this review ; but 

 comijaring these " families " with the " cohorts " adopted by Pro- 

 fessor Henfrey in the ' Micrographic Dictionary,' and with the 

 " groups " of Mr! Carruthers * in Dr. Gray's ' Handbook,' we find 

 little substantial difference. 



With regard to the last family, the Actiniscece, it has been 

 questioned whether any of the genera there included, viz. Dk- 

 t>/oc7ic(, Actiniscus, Mesocena, and Eucampia, ought to be included in 

 the Diatomacece. Dictyocha and Mesocena have been supposed to be 

 spicules of Echinodermata, and Eucampia has been placed by Kiit- 

 zing in the Desmidiacece, and by Smith in the Diatomaceoi. Dr. 

 Rabenhorst admits that in habit and structure they differ widely from 

 all Diatomacece, but he considers that, having regard to their siliceous 

 covering, they ought not to be excluded from the class. 



Dr. Rabenhorst's second class is the Phycochromophycece. It has 

 been remarked above that there will probably be no objection raised 

 to the separation of the Diatomacece from the other freshwater 

 Algfe ; but the same can hardly be said of the class Phycocliromo- 

 phycece, which can only be looked upon as temporary. 



The nature of phycochrom is not yet very well understood. The 

 term was invented by Niigeli in his ' Einzellige Algen,' where, 

 after stating that in most of the unicellular Algae the colouring- 

 matter is chlorophyll, he says : — 



" la other genera of unicellular Algc'B, especially in the Chroococcacece, 

 the cell-contents exhibit a peculiar colouring-matter.... It is usually 

 bluish green (verdigris-green), very often orange or brick-red ; sometimes 

 it is violet- or copper-coloured, very rarely blue, yellow, or pure i-ed." 



Cohn, in speaking of the Oscillannece, says that the verdigris- 

 green colouring-matter of these plants, the phycochrom of Niigeli, 

 is a compound body, consisting of a green substance insoluble in 

 water, but soluble in alcohol and ether, viz. chlorophyll, and of a 

 substance (conversely) soluble in water and insoluble in alcohol and 

 ether, which he calls j:)7(yco(^yfOi. 



He says that, in living cells, both colouring-matters combine to 

 form a compound colour, the phycochrom of Xiigeli. Dr. Aske- 

 nasy, in his papers in the ' Botanische Zeitung'f, discusses the 

 remarkable optical properties (fluorescence and the bands of ab- 

 sorption produced in the spectrum) which are exhibited by chloro- 

 phyll, and by the colouring-matter oitheFloridece, oiPeUigera canina, 

 and of Collema ; and at the conclusion of his remarks he says — 



" With regard to the names phycochrom of Nageli and rhodophyll X 

 of Cohn, I believe that they are now superfluous; for they signify 



* Mr. Carruthers's arrangement is that of Ralfs, with some modifica- 

 tions by Meneghini, Kiitzing, and others. 



t " Beitriige zur Kenntniss des Chlorophyll und einigen dasselbe be- 

 gleiteuder Farbstofte," Bot. Zeit. July 19 & 26, 1867. 



X Cohn's " rhodophyU " is the reddish-brown colouring-matter of the 

 Flo^'idece. 



