60 COCCOPHYCE.E. 



young plants, and on the other, either to destroy the vitality of the 

 whole colony or to drive it to seek ref age in deeper water. 



" A curious instance of this sensitiveness to varying conditions of light 

 and heat occurred to myself. I had two shallow vessels in a north 

 window, each containing a goodly supply of Volvox. Cold and inclement 

 weather, which prevailed for weeks together, seemed to check their 

 increase, for I found but few young spheres from day to day among the 

 older ones. Thinking that a moderate degree of warmth would tend 

 to increase my colony, I transferred one vessel, fortunately not both, to 

 the floor of a warm greenhouse. In forty-eight hours all were dead, and 

 in a few days scarcely a vestige remained of the countless corpses which 

 had copiously strewed the bottom of the glass. 



" We must now revert to the minute structure of the mature parent- 

 sphere, which has been exhaustively studied by Cohn, Busk, and 

 Williamson. 



" In the outset it should be stated that the last-named observer 

 believes that there are two distinct forms of Volvox, in one of which 

 the peculiar structure which I am about to describe exists, while it is 

 absent from the other. Busk disputed the accuracy of Williamson's 

 observations on this point, but in an appendix published subsequent to 

 the body of his essay he states that he has detected this same structure 

 in specimens from Manchester, but not in his own. 



" I have failed to develop it by the means recommended by William- 

 son, but have succeeded in making it evident enough in a great number 

 of specimens from. Sntton, by the use of these reagents, and especially 

 by the application of aniline purple, an invaluable auxiliary in the 

 examination of minute vegetable cell-structures. 



" This substance stains the protoplasmic elements of such structures 

 to a colour which appears deep purple by direct light and crimson by 

 dark background illumination, and reveals details which are wholly 

 invisible without its use. 



" The colour is, however, greedily absorbed by some of the materials 

 used by the microscopist, so that a judicious choice of these is necessary 

 to ensure success. Objects stained in this manner are, for instance, 

 rapidly bleached if mounted in gold -size cells, and I have for the present 

 adopted zinc-white in its place. Among other reagents which I have 

 used are eosin, iodine, iodised glycerine, carmine solution, potassium 

 permanganate, nitrate of silver, and other salts, some of which bring 

 into view various parts of the minute structure of plants ; but aniline 

 colours, applied with due precautions, produce the most rapid and 

 striking effect. 



" Professor Williamson describes the structure in question as a net- 

 work of lines dividing the whole surface into hexagons, in the centre of 

 each of which is seated one of the gonidia. 



" The delicate ' protoplasm-threads ' proceeding from each of these 

 to its six surrounding neighbours never pass through the angles of the 

 hexagons, but always through the side of each hexagon to the next 

 gonidium. (Plate 23, Fig. 3.) Hence it appears that 'the points of 

 alhesion are chosen prior to the development of the outer cell 

 membrane,' in which light Williamson regards the hexagonal division. 

 In his specimens this structure was developed by immersion in glycerine 

 for some time. I have failed to obtain more than the faintest sugges- 

 tion of it by these means, but it is often brought out by the application 

 of aniline purple, as is also an important detail shown in drawings 

 made from his preparations, viz., that at the angles of the contiguous 

 hexagons there is sometimes a distinct doubling or separation of the 

 lines, whence he concludes that each side of the figure is really formed 

 by two delicate cell-walls in close juxtaposition, the duality of which is 



