LICHENS. 309 



The teleutospores of the Rust (Puccinia arenariae) found on Chick- 

 weed and other Caryophyllaceae, e.g. Sweet William, germinate as 

 soon as ripe, hence they serve admirably for the study of the stages 

 which may not be readily observed in Wheat Rust. 



XANTHORIA PARIETINA. 



447, General Characters. This Lichen (which was 

 formerly placed in the genera Parmelia and Physcia) is 

 very common on roofs, old walls, trees, etc., often forming 

 large brilliant orange patches. 



Examine a patch, and note (1) that the thallus is leaf- 

 like or foliaceous, smooth, bright yellow above but pale and 

 whitish below ; (2) the irregular branching lobes at the 

 margin of the thallus, which tends to assume a more or 

 less circular outline ; (3) that the margins are free and can 

 be raised from the substratum by means of a knife, with- 

 out damaging any tissue, but elsewhere it is firmly attached 

 by (4) whitish processes called rhiziiies ; (5) on the upper 

 side, the small cup-like organs or apothecia, each apo- 

 thecium being about 3 or 4 mm. in diameter and bright 

 orange in colour ; (6) that the thallus is brittle when dry, 

 but when moist or after being soaked in water it becomes 

 soft though leathery in texture. 



448. T. S. of Thallns (Fig. 66). Cut transverse sec- 

 tions across a part not bearing apothecia ; place the sections 

 in water and note that they swell. Mount some sections 

 in glycerine and note 



(1) The upper yellow limiting or epidermoid layer, 

 not sharply marked off from 



(2) The upper cortex of densely aggregated hyphae, 

 forming a pseudo-parenchyma tissue of cells with thick 

 swollen walls and scanty contents. 



(3) The colour in the outer layers is due to crystalline 

 yellow granules deposited between the hyphae and also 

 on the free upper surface of the thallus. 



(4) The broad medullary zone of loosely interwoven 

 hyphae. 



(5) The large green Alga cells (" gonidia "), either iso- 



