138 ASCOLICHENES. 



come gelatinous when exposed to moisture (Fig. 132), on account 

 of the swelling of the walls of the Alga?. The hyphas contain 

 protoplasm with drops of oil, but never starch ; their walls easily 

 swell when exposed to damp after having been dried, and in some 

 (e.g. Cetraria islandica) they become gelatinous when cooked. 

 Certain strata of hyphas become blue on treatment with iodine 

 alone, from which it is inferred that the wall is allied, in its 

 chemical nature, to starch. 



2. The enclosed Algae, termed " gonidia." Some belonging to 

 the Cyanophycese, Protococcoidese, (especially Pleurococcus) and 



FIG. 132. Collema microphyllum. Transverse section through the thallus ; g Nostoc- 

 chains; h hyphse. 



Chroococcacese, are spherical and are found isolated, or in irregular 

 groups of cells (Fig. 131 g) ; some belonging to Nostoc (Fig. 

 132 </), Lyngbyaceae, etc., are placed in cell-rows. Each Lichen, 

 as a rule, has only one definite Algal-form for its gonidium. 



The gonidia either lie together in a certain stratum between 

 the cortex and the medullary layer (Fig. 131 g}, or are scattered 

 irregularly throughout the entire thallus (Fig. 132). The thallus 

 is in the first instance termed " heteromerous," in the second in- 

 stance, "homoiomerous." The Fungal-hyphje embrace the gonidia 

 and apply themselves closely to, or even penetrate them, and 

 hence it has been difficult to decide whether the one cellular form 

 does or does not develope from the other (Figs. 134, 135). 



