498 MESSRS. R. PAULSON AND S. HASTINGS ON THE 
more definite information as to the frequency of penetration, if any, in some 
of our common lichens. For the purpose we have used as material 
Ramalina calicaris, R. farinacea, Usnea florida, Platysma glaucum, Evernia 
prunastri, Parmelia sawatilis, P. sulcata, P. caperata, P. acetabulum, P. fuli- 
ginosa, P. physodes, Xanthoria parietina, Physcia pulverulenta, Cladonia 
pyvidata, C. coccifera, C. digitata, C. macilenta and C. sylvatica. Our speci- 
mens were collected mostly in the Home Counties, either during the months 
of February and March or through the autumn months of September and 
October. Lichens are probably at their most active period of growth, in the 
south-eastern part of this country, during the early months of the year, for 
it has been found by us, that the process of multiplication of the algal cells 
is exceedingly active during that season of the year; so much is this the 
case, that, in growing portions of the thallus the number of gonidia under- 
going change is so great that full-sized cells, in a vegetative state, are 
comparatively few (PI. 21. phot. 1). 
Sections eut from fresh material were examined in water io which 
» per cent. of pure glycerine had been added, together with a small quantity 
of 75 per cent. alcohol. Other material was fixed either with absolute 
alcohol or in a chromic acid solution, prepared according to Schaffner’s 
formula, viz., chromic acid 3 gr., glacial acetic acid 3 c.c., water 100 c.c. 
This latter gave excellent results with little or no shrinkage. Small portions 
were left in the solution for 20 to 24 hours and were then thoroughly 
washed in distilled water for at least 12 hours. Fixed material was 
embedded in paraffin and cut with a microtome in ribbons from 2 to Tu in 
thickness. 
Methyl green was found to be a most useful stain for fresh material as 
the fungus becomes blue and stands out in contrast with the alga which 
remains green. For fixed material we selected Donney's (1) staining 
medium, Haidenhain's iron alum-heematoxylin and eyanin and erythrocin, as 
differential stains. 
Bonney’s triple stain was tried as an experiment, for it was previously 
known to us as a stain for animal tissues only. Our object was to find 
a reliable reagent that would clearly differentiate the alga and fungus, and 
also show distinctly the various parts of the algal cell and the hyphal thread. 
With this medium the algal cells become orange-coloured, the central so-called 
pyrenoid (nucleus ?) reddish violet, and a small lateral body, surrounded bya 
light area, is stained a dark purple. The hyphal protoplasm also becomes 
purple. Many of the younger gonidial cells are not stained in the same 
manner as the mature cells; the latter, as we have already stated, become 
orange-coloured, while the former stain dark purple. We suggest that the 
difference in colour is due to the fact that the large cells are cut through 
when sections are made, while many of the smallest cells remain entire, and 
the colour which comes out so readily from cut cells, when washing the 
