400 DIVISION III. MODE OF LIFE OF THE FUNGT. 



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. 



