242 Poulton . — The Structure and Life-history of 
this was further surrounded by a colourless layer consisting of interlacing 
fungal hyphae without algae. This is presumably the youngest portion of 
the thallus. 
Minute Structure of Thallus. 
The thallus exhibits considerable diversity of structure. A vertical 
section of the young thallus will reveal a large number of green algal cells 
scattered irregularly throughout a colourless network of fungal hyphae 
(Fig. 3). The alga is a form of Frotococcus viridis , Ag. ( = Pleurococcus vul- 
garis auct.). 1 Each cell is furnished with a lobed parietal chloroplast ; no 
pyrenoids have been detected. The algae occur either singly or in groups of 
two or three cells. If the thallus is very young, the algae are chiefly unicellu- 
lar, and many of the cells are somewhat elongated, preparatory to division. 
The fungal hyphae are absolutely uniform in structure, with rather short 
cells. In horizontal and vertical sections of the thallus, and in surface view, 
the hyphae have the appearance of a compact pseudo-parenchymatous 
tissue. There is thus no definite arrangement of the alga with reference to 
the hyphal network ; the thallus is strictly homoiomerous. 
The older thallus exhibits an interesting structure. The algae are often 
disposed in short chains of cells (3-7 or more) at right angles to the surface 
of the thallus. In many cases the cells are connected to form a continuous 
chain, suggesting that they result from the division of a single algal cell 
(Fig. 4). A controlling factor in the distribution of the algal cells may be 
the excessive amount of moisture to which the algae are subjected. Proto- 
coccus cells frequently produce short filaments when immersed in water or 
culture solutions. (See Cultures, p. 245.) 
A further type of thallus-structure has been observed — e. g. there is • 
sometimes a rudimentary distinction into layers, short chains of algal cells 
occurring aggregated near the dorsal and ventral surfaces of the thallus. 
Separating these is a conspicuous layer of fungal hyphae, entirely devoid of 
algae. The hyphae are interwoven to form a pseudo-parenchymatous tissue 
absolutely identical and continuous with that in which the algae are embedded 
(Fig. 5). This type of structure has been observed on the same thallus as 
the homoiomerous type. 
I have not succeeded in detecting the presence of haustoria connecting 
the hyphae with the algal cells. Owing to the close juxtaposition of the two 
constituents it is presumable that interchange of food materials takes place 
between alga and fungus, by osmosis through the cell walls. 
An interesting feature of the thallus is the relatively great abundance 
of the alga. Green lichens are somewhat rare, and in the present instance, 
1 The structure of the alga of the thallus, and its behaviour in culture solutions, are in precise 
agreement with Chodat’s Pleurococcus Nagelii. (Polymorphsme des Algues, pp. 53 et seqq.) 
