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504 MESSRS. R. PAULSON AND 8, HASTINGS ON THE 
out the culture method, we present the result of observations based alone 
upon the microscopic image. 
West (9) says: * A few of the Chlorophyceœ have become constituents of 
the thalli of many lichens .....— It is mostly one or two of the unicellular 
and colonial members of the Protococeacer whieh are thus found.” In the 
same work he states “ Chlorococeum, Fries, 1825 [= Cystococeus, Nüg., 
1849],” so that the name so long associated with the gonidia of lichens is 
now Chlorococcum humicola. 
As a result of the present investigation, we have arrived at the conclusion 
that the gonidium of the lichens mentioned in this paper is most probably 
a species of Chlorella, and that the daughter gonidia are reduced zoogonidia. 
West (9) suggests that this is not unlikely the explanation of the similar 
form of reproduction in the case of free Chlorella. We do not attempt to 
name the species that forms the algal symbiont, but it is perhaps worth 
noting that the mature gonidium of the lichens already referred to has a 
greater diameter than that of a free cell of Chlorella vulgaris, Beijer., and 
that during the formation of daughter gonidia, the number of such produced 
within the mother cell of the lichen gonidium is larger than the number 
formed in the free cell. 
There is reason for concluding that Protococeus viridis, Ag.. forms the 
gonidium of very few of the common British fruticose and foliose lichens, 
for we have not met with any vegetative cell-division whatever among the 
gonidia of the material that we used. 
While engaged on that part of our work that required magnification 
of from 800 to 1000 diameters, we have used a Zeiss 2 mm. achromatic oil- 
immersion objective with an achromatic oil-condenser. 
The photographs were voluntarily prepared for us by Mr. J. H. Pledge, 
F.R.M.S., who had at his disposal, by permission of the management, the 
whole of the apparatus of the physical laboratory of the Kodak Company’s 
factory at Harrow. He employed a Leitz 2mm. apochromatie oil-immersion 
objective, a Beek oil-immersion condenser, Watson * Holos” oculars, x5, 
x 10, and sometimes a Zeiss x2 projection eyepiece. Colour screens were 
used to accentuate or suppress certain details of the preparation. We gladly 
avail ourselves of this opportunity for expressing our indebtedness to him 
for the skilful care exercised in the production of the photo-mierographs. 
We take this opportunity of thanking Professor G. S. West, who most 
generously gave us advice and kindly examined some of the preparations 
from whieh the photo-mierographs had been taken. 
Summary. 
A summary of the investigation may be made by reference to Cladonia 
digitata, var. denticulata Ach. var. monstrosa Nyl., the lichen which has been 
used as material for many of our preparations, 
