738 LICHENES. 



and mode of division of the thallus vary greatly, so that many varieties 

 of the plant have been distinguished. 



In the dry state, Iceland moss is light, harsh and springy ; it absorbs 

 water in which it is placed to the extent of a third of its weight, be- 

 coming soft and cartilaginous; it ordinarily contains about 10 per cent, 

 of hygroscopic water. It is inodorous, but when wetted has a slight 

 seaweed-like smell ; its taste is slightly bitter. 



Microscopic Structure — A transverse section exhibits, when 

 strongly magnified, a broad loose central layer of long, thick-walled 

 branching walls of hyphce, containing air, and enclosing wide hollow 

 spaces. This middle layer encloses a certain number of larger cells 

 called gonidia, coloured with chlorophyll. The gonidia are not destroyed 

 either by strong sulphuric acid, or by boiling them with potash. They 

 assume however a deep violet colour when treated with caustic potash 

 and then left for 24* hours in a solution of iodine in potassium iodide. 



The tissues on either side of this central layer consists of very 

 thickly felted hyphse, without intervening spaces, and does not appear 

 to contain any particular substance. This compact and tenacious 

 tissue passes into a thin cortical layer consisting of cells very closely 

 bound together. Under the influence of reagents this layer becomes 

 very evident: thus when moistened with strong sulphuric or hydrochloric 

 acid, it separates from the rest of the tissue as a coherent membrane, 

 and rolls itself backward. On boiling with water the inner tissue swells 

 up, the cell-walls being partly dissolved. Thin slices of the lichen are 

 coloured reddish or pale blue by iodine water, — more distinctly blue, if 

 previously treated with sulphuric acid. The colour spreads uniformly 

 over the inner tissue, but no starch granules can be detected ; the cortical 

 layer is merely coloured brown by iodine. The white spots on the outer 

 surface of the thallus are resolved by pressure under a plate of glass 

 into minute round transparent granules, not coloured by iodine, and 

 thick branched cells like those of the central layer. 



The short thick prominences on the edge of the thallus, frequently 

 terminate in one or more sac-like cavities {spei^TYiogonia) containing a 

 large number of simple bar-shaped cells {sperrtiatia), only 6 mkm. long ; 

 they are enveloped in transparent mucus, and may be expelled by 

 pressure under glass. It has been shown by Stahl (1874) that they repre- 

 sent the fertilizing corpuscles or seaweeds of the class FloridecB. 



The observations of De Bary (1866) and Schwendener (1867-70) 

 confirmed and much extended by the researches of Bornet^ (1873-74), 

 have shown that the gonidia of lichens are referable to some 

 species of Alga, and are capable of an independent existence; that the 

 relations of the hyphse to the gonidia are of such a nature as to exclude 

 the possibility of either of those bodies being produced by the other ; 

 and further that the theory of parasitism is the only one capable of 

 explaining these relations in a satisfactory manner. Under this singular 

 theory, lichens are compound organisms, formed of an alga, and of a 

 fungus living upon it as a parasite. 



Chemical Composition — Boiling water extracts from Iceland 



^ Recherches sur lea gonidies des Lichens.— — For a complete abstract of these and all 

 Ann. den Sciences nat. Bot. xvii. (1873) the more recent investigations on this sub- 

 45-110; 11 plates ; alsoxix. (1874) 314-320. ject, see Luerssen (I.e.) 186 et seq. 



