CHAPTER VII. — PHENOMENA OF VEGETATION. — LICHENS. 407 



medulla and the margin surrounded by a 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 

 cyphellae ; 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 



