Sporulation of Lichen Gonidia. R. Paulson 45 
of the divided protoplast are clearly indicated, but in the latter 
case there is no cell-wall between the masses. 
Bornet (2) in his illustrations of the synthetic building of the 
thallus of Physcia (Xanthoria) parietina from the germinating 
fungus spores of that lichen and the cells of Protococcus leaves 
one in doubt respecting the identity of the alga. On PI. X, fig. 2 
there are a few cells that indicate division by transverse walls; 
this suggests Protococcus viridis Ag. but great uncertainty on 
this point arises when it is known that the gonidium of Physcia 
paretina within the thallus, and when isolated, does not divide 
by a transverse wall but that sporulation takes place similar 
to that described respecting E. prunasivi. The question must 
arise as to whether the alga used in the synthetic building of 
Physcia parietina was pure, in the modern sense of the word. 
When cutting sections of a lichen thallus a few cells dividing 
vegetatively are often present on the microscope slide; they 
are stray ones that have settled upon the upper surface together 
with fungus spores and other foreign matter. It is of great 
importance that the thallus should be thoroughly washed before 
commencing to cut sections for examination, for without this 
precaution erroneous conclusions will most probably be the 
result. 
In a recent paper(7) it was suggested that the gonidium 
common to many lichens is a species of Chlorella, from the fact 
that the sporulation of the algal cells, within the thallus and 
also in those cells that have been isolated from it and subjected 
to cultural methods, is exactly similar to that which takes 
place in free Chlorella cells. The number of daughter cells pro- 
duced in the green gonidium of the lichen thallus is usually 
eight or sixteen. In some free species of Chlorella, viz. C. miniata 
Kutz. the same numbers occur. Isolated gonidia, from the 
thallus of Cladonia pyxidata, cultivated on agar-glycose, pro- 
duced as many as 32 daughter gonidia and in one case a zoospore 
appeared (No. 63 de la Collection Chodat (3)). Gonidia containing 
eight or sixteen daughter cells frequently occur within the 
thallus of Cladoma pyxidata 
The identification of the green alga which is common to a 
large number of lichens has presented great difficulties in the 
past. Opinion has been based upon the assumption that the 
absence of spore formation in the lichen thallus is owing to the 
fact that the alga, on becoming a gonidium, is profoundly 
altered, so that it ceases to produce aplanospores or zoospores 
or both. As spore formation of the gonidium has now been 
clearly demonstrated (figs. 2, 5 and 6) the standpoint as to the 
identification of the alga is completely changed. The weight 
of evidence is in favour of the alga being a species of Chlorella. 
