RELATION BETWEEN THE ALGA AND FUNGUS OF A LICHEN. 503 
to remain constant in a large number of lichens. On the upper surface, the 
hyphæ anastomose and become so closely welded together that the separate 
threads cannot be recognised, and a false tissue results (Pl. 21. Phot. 3). 
The hyphz of the gonidial layer have thin walls, they branch frequently, 
and are shortly jointed. These short branches, in contact with algal cells, 
swell out considerably into pyriform ends, and present a large surface of 
contact with the cells that they surround (Pl. 21. Phot. 3). Sometimes a 
gonidium is attached to the end of one of the short hyphal branches and 
looks as if it had been cut off from that branch. The reagent Cl.Zn.I. 
differentiates the cell-wall of the spherical cell from that of the branch. 
The hyphe of the medulla form a loose felt with abundant air spaces 
(Pl. 21. Phot. 1). 
We have noted two forms only of contaet between hypha and gonidium : 
(1) when the gonidium is surrounded with loosely-applied hyphæ, “ the 
simple contact " of Schneider, (2) the intimate contact where, by a modifi- 
cation in shape, the hypha presents a large amount of surface to the gonidium 
to which it attaches itself. Such branches are known as extra-cellular 
haustoria. We have not found a ease that could be regarded as definite 
hyphal penetration of a living cell by intra-cellular haustoria. From this it 
follows that the occurrence of fine gelatinous filaments, protruding from a 
penetrating hypha and forming a network surrounding and even penetrating 
the chloroplast, have not been seen, consequently we are unable to accept 
any theory based solely upon their existence. Cases sometimes occur which 
resemble a network of threads around the chloroplast. The appearance is 
due occasionally to plasmolysis, and can be produced by placing sections of 
fresh material in pure glycerine. The beautiful reticulation of the cyto- 
plasm (PI. 21. Phot. 2} also produces the appearance of a network of threads 
under certain conditions of the microscopical illumination of the object. 
It has already been mentioned that the mature chloroplast is a spherical 
body covered with minute verrucose protuberances. A minute object with 
such a structure is likely to give rise to the formation of a defective image if 
the illumination from the substage condenser of the microscope is not perfectly 
centred, and when there is a misprision of focus. A defective image may 
also be produced by using a smaller cone of light than that required by the 
numerical aperture of the microscope objective in use. The defective image 
in each case gives more or less the appearance of threads passing over the 
chloroplast. 
The condition of the protoplast of the gonidium, just previous to the 
formation of spores, presents features that are difficult to explain, but no 
penetrating hyphz have been found in such cells. 
With the microscope alone it is extremely difficult to decide even the 
family of the Chlorophyces to which the green spherical gonidium, common 
to a large number of lichens belongs, and as we have not attempted to carry 
