THE GENUS HYDRODICTYON. 57 



like a Protococcus cell. After a size of about T T -th m.m. is 

 attained, the endochrome divides successively into several portions. 

 The external layers of the surrounding wall now give way in some 

 spot, and allow the inner layers to protrude and form a sort of 

 hernial sac, into which the several endochrome masses soon pass, at 

 the same time assuming the well-known characters of true zoospores. 

 Froin two to five of these bodies are thus produced out of each 

 original microgonidium. They are large, ovate, biciliate, and, 

 generally, soon escaping from the hernial sac, move about actively in 

 the water for a few minutes. Sometimes, however, they set lie 

 down within the generative utricle. In either case, after a little 

 time, they become motionless, lose their cilia, and develop into 

 polyhedral cells, which are structurally remarkable for havingtheir 

 angles prolonged into long horn-like appendages. Under favour- 

 able circumstances, at the end of a few days, the bright green 

 endochrome of these undergoes similar changes to those described 

 as presaging the production of the microgonidia, and is finally 

 formed into zoospores, which, in from twenty to forty minutes, 

 unite, within the polyhedron or large cells, into a Jlydrodictyon, 

 which is finally set free by a solution of the cellulose coat of the 

 polyhedron. The network thus formed differs in no essential way 

 from tli at which arises in the better known way, except that it is 

 composed of much fewer cells. It is generally a closed sac ; but 

 when the polyhedron, out of which it is developed, is small, it is 

 sometimes merely an open network. Its after history appears to be 

 identical with that of the ordinary Hydro diet yon frond. 



See also Micrographic Dictionary, under Hydrodictyon — and 

 HassaH's British Fresh Water Alga?, p. 225, plate lviii. 



LABRELLA PTARMIC/E. 



The following is the description of a parasite on Achillcea ptarmica 

 gathered at Ealing, and sent for determination by G. D. Brown, 

 as probably a species of Phacidium, which, externally, it greatly 

 resembles : — 



Labxella Ptaxmicae. Desm. PI. Crypt Xo. 189. Fries Elen. ii. 

 149. Schizothyrium Ptarmica?, Desm. Ann. des Sci. Nat. xi. (1849), 

 p. 560. Kickx. Crypt. Flor. Fland. i., p. 4C7. 



Innate, black, somewhat shining, rounded or ovate ; asci cylin- 

 drical or subclavate ; sporidia minute, ovoid. 



On leaves and stems of Achillcea Ptarmica. 



The Rev. M. J. Berkeley observed this many years since on in- 

 fected plants of Achillcea which he brought home from France, and 

 cultivated in England ; but it was not included in the British Flora, 

 as the fungus was manifestly brought over with its host. This 

 may also be the case in the present instance. Schizothyrium is per- 

 haps the most accurate determination, but the fungus is better known 

 as Labrella. Tin- former genus is classed with the Phacidiacece. 



