502 MESSRS. R, PAULSON AND S. HASTINGS ON THE 
plasm of the hyphal thread, but this is owing to a difference in permeability 
of the dead and living cell-walls, for the contents of a living cell are stained 
similarly if left in the stain for a few days 
À large central so-called pyrenoid with a diameter of about one-third that 
of the cell, viz., 3 to 5, is almost invariably present (Pl. 21. Phot. 1, ».). 
It is there, in the newly-formed gonidia, at a very early stage, when the 
diameter of the gonidium does not exceed 2? u. It is readily seen in. gonidia 
from fresh material, and stains very quiekly in that which has been fixed, so 
much so, that it becomes a prominent feature in all stained preparations 
except in cells about to sporulate, and in the separated masses of the 
original protoplast immediately after division. We regard it as the nucleus 
of the cell. 
With Bonney’s or Haidenhain's stain the so-called pyrenoid (PI. 21. 
Phot. 1, n.) shows a definite structure, not merely on the circumference, 
where. it might be interpreted as being due to amyloid substance, but a 
structure that runs throughout the whole body and is seen in all sections in 
whatever direction the cut may be made. It certainly has not a distinct 
erystalline form such as that which is often figured. 
In some sections there are cells which we have named twin gonidia 
(Pl. 21. Phot. 2). It appears that two of the eight or more daughter cells, 
while still within the mother cell-wall, instead of secreting each its own cell- 
wall become surrounded by a common boundary. It does not appear probable, 
after the close examination of many preparations, that division of the proto- 
plast into two portions only takes place. These twin gonidia are mostly 
equal or nearly equal in size, but occasionally one is normal while the other 
is crushed against the common cell-wall during the carly stages of develop- 
ment and so remains quite small. 
We regard the formation of daughter cells as a perfectly natural process 
taking place abundantly at a definite period of the year. Danilov suggests 
that division of the protoplast takes place after the gonidium has been pene- 
trated, and that the change is probably an effort to ensure that some of the 
protoplast of the invaded cell should escape contamination by the fungus 
plasm. The great activity in the formation of spores at a certain datids 
season of dis; year leads us to the conclusion that the formation of spores 
corresponds to the similar process that takes place when isolated gonidia are 
cultivated upon agar-glucose and are not subjected to any excitement from 
fungoid hyphe. 
The diameter of the hyphal threads of lichens varies with the species, 
much more than does that of the diameter of the gonidia. Taking the 
hypha of the medulla, that part where the hypha is the least modified in the 
lichen thallus, we find that the diameter varies from 3 to 4:5. It may be 
2p in one species, 45 in another, while the mature gonidia have the same 
average size in both. The symbiont fungus differs, while the alga appears 
