116 PLANT-LIFE 



500 diameters. The upper and lower layers, t and u, 

 consist of dense tissue elaborated from the hyphae of 

 the Fungus-partner. The captive Algse, some of which 

 are shown increasing by division, are indicated by a. 

 It will be observed that they are situated in what is 

 known as the " medullary zone," where the hyphse are 

 loosely arranged and there are plentiful air-spaces. 

 They are, also, in the upper part of this zone, and, 

 therefore, nearer the upper surface of the thallus than 

 the lower. Thus, the Fungus which holds the Algse 

 captive takes care that they shall benefit by light, and 

 have a sufficiency of air; otherwise they would not be 

 able to form carbonaceous food, and so supply their 

 captors with nourishment. In Physcia parietina the 

 Algal-partners are of the one-cell species, Cystococcus 

 humicola, and closely related to Pleurococcus, described 

 on p. 55. 



The fruits of this Lichen are indicated by a in Fig. 38. 

 They, in common with the fruits of the Discomycetes 

 generally, are distinguished by the term " apothecia " 

 (Gr. apotheke, a storehouse). In the same figure, h 

 represents a somewhat magnified apothecium, seen in 

 section, with the exposed hymenial surface. The fruits 

 are easily seen by the naked eye; they are deeper in 

 colour than the thallus as a whole, and appear as flat 

 discs surrounded with rims at their edges. A highly 

 magnified vertical section of an apothecium is shown in 

 Fig. 40. Above we note the paraphyses, or sterile hairs, 

 p, the asci, a, two of which contain eight spores, sp, 

 each. The whole surface of the disc within ,the rim is 

 covered by the'thedum, as the layer containing the asci 

 and paraphyses is called. The asci, of course, are very 



