312 LICHENS. 



451. Examine some ascospores with the high power, 

 and note that each spore is two-celled, with a peculiar 

 structure the two rounded cells are at the two poles of the 

 spore, and they are connected by a protoplasmic strand, the 

 wall of the spore being of great thickness except at the two 

 ends. 



452. Spermogonium and Spermatia. In some of 

 the sections the spermogonia (very similar to those of 

 Puccinia) may be seen ; but our type does not produce 

 spermogonia freely, and these organs are better seen in the 

 so-called " Iceland Moss " (Cetraria), which can be bought 

 dried from a druggist, or in the " Reindeer Moss " (Cladina), 

 which grows commonly on heaths. In Cetraria, cut sections 

 passing through the little marginal teeth of the thallus ; 

 in Cladina, cut sections of the drooping tips of the erect 

 much-branched thallus. Note that the spermogonia are 

 flask-like cavities containing numerous converging fungal 

 hyphae from which are abstracted the small unicellular 

 spermatia. 



453. Soredia. Our type is usually fertile, with abundant apo- 

 thecia, but some specimens may be found with few or no apothecia, 

 and these are likely to bear soredia. Remove these by scraping the 

 upper surface of the thallus on to a drop of water ; or moisten the 

 thallus and press it on a slide. The soredia are rounded bodies, 

 each soredium consisting of Fungal hyphae enclosing a few Alga- 

 cells. Soredia can be obtained in great numbers on various species 

 of Cladonia the so-called "Trumpet mosses" which are very 

 common; the whole "trumpet" or stalked cup-like structure 

 (podetium) is often covered with a greyish-green powder consisting 

 of soredia, while the apothecia form brown or in one species ("red 

 cup-moss "or " matches ") scarlet outgrowths on the margin of the 

 cup. 



COLLEMA PULPOSUM. 



454. General Characters. The family to which 

 Collema belongs is distinguished from the majority of 

 other Lichens in that (1) the thallus is extremely gela- 

 tinous ; (2) the form of the thallus is determined by the 

 Alga, not by the Fung us; (3) the thallus is homoiomerous, 

 i.e. the Alga and Fungus are distributed uniformly through 



