400 DIVISION IIL—MODE OF LIFE OF THE FUNGI, 
Lecanora pallida commences its growth according to Frank’s observations with 
the development of a hyphal thallus in the periderm like the Graphideae just 
mentioned, while the Alga which belongs to it, a form of Cystococcus (?) with free 
round cells, comes ‘casually’ into contact with the hyphal thallus, its cells being 
caught in the superficial crevices of the periderm in which it is spreading and there 
seized upon by the Fungus. This is the origin no doubt of many Lichens which 
occur in the same situation. 
The nicest adaptation between Alga and Fungus, and one which goes so far as to 
exclude ‘accident,’ is displayed in a small number of species of Lichens which Stahl 
especially has closely examined: Stigmatomma cataleptum, Endocarpon pusillum, 
Polyblastia rugulosa, etc. Here the cells of the Alga departing from their usual 
behaviour make their way regularly into the hymenium of the sporocarps of the 
Fungus, and increase to such an extent between the paraphyses, that they lie in great 
quantities round the asci as they ripen. Whenever any spores are discharged a 
certain number of algal cells adhere to them and are ejected with them from the 
sporocarp, and are ready at once to be seized upon by the germ-tubes. 
The morphological characters of the Alga when it is assailed by the Fungus 
experience a more or less profound change from its condition in the free state. Size; 
structure and arrangement of cells are affected in a manner which varies in the same 
species of Alga according to the species of Fungus to which it serves as host. In 
this point also specific differences which are noticed in descriptive works on the 
Lichens make themselves apparent in different cases. We will only mention here as 
an example of these differences the smooth round gelatinous colonies of Nostoc 
lichenoides and similar Algae, which are changed by the intrusive Fungus into lobed 
thallus-bodies with progressive growth in the direction of the margin, but usually without 
the structure and arrangement of the algal cells which they contain being affected. 
With these may be compared the hymenial Algae of Stahl just mentioned. These in 
Endocarpon pusillum are a form of Pleurococcus. Within the hymenium the cells 
of the Alga are very small, roundish in form and a pale green, and they multiply by 
successive bipartitions, the new cells separating from one another. When seized by 
the hyphae of Endocarpon their diameter increases to six times the original size, their 
parietal chlorophyll-corpuscles assume a bright green colour, and the bipartitions 
are continued with irregular arrangement of the successive partition-walls and a 
corresponding change in the grouping of the cells themselves. The Alga retains 
these characters as the thallus developes. If the same Pleurococcus-cells when ejected 
from the hymenium of Endocarpon pusillum are seized upon by the hyphae of 
Thelidium minutulum, they assume the same green colour, but they do not increase 
so much in size, and within the surrounding hyphal weft the divisions take place by 
partition-walls disposed alternately in three directions and cutting one another at 
right angles. In accordance with these divisions the cells are arranged in rect- 
angular packets with blunt angles and divided into squares, just as they are arranged 
when vegetating free and alone (Fig. 170, 175). 
Lastly it is of importance to call attention to a fact noticed by Frank in the 
Lichens mentioned above as studied by him, that a part of the algal cells in the 
Lichen may to a great extent or entirely lose their chlorophyll without apparently 
dying in consequence of the loss; but this point requires closer investigation. 
