CHAPTER VII.—PHENOMENA OF VEGETATION.—LICHENS. 407 
medulla and the margin surrounded bya raised portion of the rind. The pits are formed 
according to Schwendener by the cortical layer being first thrust outwards in the form of 
a wart by exuberant growth of the medulla and then ceasing to grow in the pro- 
tuberant part, while the surface of the thallus continues to enlarge all round it. In 
some species, as Sticta macrophylla, the interruption in the rind is preceded by the forma- 
tion of a hollow space in the enlargement in the medulla. The pits are termed 
cybhellae ; the older view, that they are small gemmae-cups, receptacles of soredia, is 
unfounded, or is true only in the case of certain species, as Sticta aurata (Schwen- 
dener). 
There is little to add to what has been already said of the crustaceous Lichens. 
Most of them are distinguished by the comparative thickness of their rind. 
The margin of many species, especially those which live in the bark of trees, is 
often marked by a black line which has been noticed by many writers ; Lecidea entero- 
leuca, Kbr. among the species specially mentioned above is an instance of this phenome- 
non, which is evidently caused by the dark colouring of the cell-membranes of the few 
uppermost layers of hyphae ; this colouring appears at a certain period in the life of 
the plant in the marginal zone where there are still no Algae. Where the thallus is 
engaged in active growth, the black line, if it is present, runs within the young colourless 
margin. The black colour is absent from the prominences which contain algal cells, 
doubtless because the few cell-layers which are so coloured are forced apart by the 
dilatation of the prominence and soon thrust aside. When the thalli of many individuals 
of the species in question come in contact with one another, the black lines often seem 
to form persistent boundaries between the different individuals like the lines on a geo- 
graphical map. The reason of this may be, that the marginal growth of the thallus 
ceases where it encounters others, and the colour then appears in the marginal zone and 
there remains (see Fig. 174). : 
The thallus of some foliaceous Lichens, Umbilicaria pustulata for instance and of 
many crustaceous forms, is capable, as we learn from Schwendener, of a growth in thick- 
ness which lasts a long time, often for years, and with this is connected a decay and 
death of the rind which advances from without inwards. The dead layer often remains 
lying on the living one as an almost structureless transparent mass ; in other cases it is 
quickly destroyed by atmospheric agencies and becomes indistinguishable. The loss 
is replaced by regeneration from the inner surface, so that the rind always remains of 
about the same thickness. The medullary hyphae whose course is in the algal zone 
ramify and intertwine and form a tissue without interstices, which becomes like the rind 
and lies close against it on the inside. The outer algal cells are inclosed in this secondary 
rind-weft, but they gradually shrivel up and die while new ones are formed inside by 
division. If the process has been going on for some time, the whole of the rind is studded 
with dying or dead algal cells. They may be shown by the cellulose-colouring of their 
membranes with iodine, especially if first treated with potash. 
The heteromerous thallus exhibits some remarkable peculiarities in respect of the 
material constitution of the hyphae. The membranes, as far as they have been exam- 
ined, show the characters of the lichenin mentioned on page 10. They are transparent, 
almost always colourless in the medulla and in the inner portions of the rind, brittle when 
dry, swelling considerably and becoming soft and flexible in cold water. Some of the 
cells of the rind at least contain air when the plant is in the dry state, and the whole 
layer is thus rendered opaque. Absorption of water expels the air and increases the 
transparency of the membranes, so that the green algal zone is seen through it ; hence 
the change of colour in the surface caused by moistening the plant. The interstices in 
the medulla continue to contain air even when moistened, and the tissue therefore 
remains opaque. The membranes of most hyphae are not coloured by iodine ; they 
dissolve on addition of sulphuric acid either without assuming any colour, as in 
Usnea barbata, or after first turning brown, as in Anaptychia ciliaris according to 
Speerschneider, or after acquiring a violet hue which is sometimes very intense. The 
