LICHENS. 311 



called Cystococcus humicola, probably allied to Pleuro- 

 coccus. Note also 



(6) The still looser lower portion of the medulla, passing 

 downwards into 



(7) The lower cortical zone, resembling the upper in 

 structure but colourless. 



(8) The rhizines or rhizoids strands of hyphae arising 

 from the lower cortex and fixing the thallus to the sub- 

 stratum. 



449. Treat sections with potash, or apply potash to the upper 

 side of the thallus in one spot and to the lower side in another, and 

 note that the tissues become reddish or purplish. Also place some 

 pieces of thallus in a test-tube, add potash and warm ; the thallus 

 changes from yellow to red or purple, this colour passing into the 

 potash solution. Neutralise by adding acetic acid ; the colour dis- 

 appears, but may reappear on again adding potash. The pigment of 

 our type resembles litmus, which is obtained from various Lichens. 



450. Vertical Section of Apothecium (Fig. 66). Cut 

 transverse sections of the thallus, passing through some 

 apothecia ; mount in glycerine. 



Note (1) the general structure of the thallus, as already 

 described ; (2) the shallow cup-like form of the mature 

 apothecium, the central portion of the upper surface being 

 only slightly concave or plane or even slightly convex ; 

 (3) the raised rim of thallus tissue around the margin of 

 the apothecium ; (4) the continuation of the Alga layer 

 into the marginal rim and also below (5) the hymenium 

 or hymenial layer consisting of closely packed vertical 

 parallel outgrowths of two kinds viz. (6) the clear para- 

 physes or sterile hyphae with thickened yellowish ends, 

 and (7) the shorter and thicker club-shaped asci ; each 

 ascus contains when mature eight ovoid spores, some of 

 which may be seen lying free on the surface of the hy- 

 menium; (8) the sub-hy menial layer, just below the 

 paraphyses and asci, consisting of densely packed hyphal 

 tissue and passing below into the looser tissue containing 

 the groups of Alga cells in its meshes (see 448). 



Treat some sections with (a) iodine, (6) chlor- zinc-iodine, 

 (c) warm water ; note the results in each case. 



