PLATE XV— ICELAND MOSS (Cetraria islandica). 



The Licheri-thallns is regarded, according to the most recent investigations, as forming not one individual organism but a kind of 

 composite structure. It is a commensal organism, formed by the partnership of different individuals preying upon, and at the same time 

 mutually accommodating, each other. The hyphae form one of the plants which is an Ascomycetous Fungus, and the gonidia or green 

 cells form another plant which belongs to the Palmellaceous Algae, in this instance, viz. — Cystococcus humicola. These Algae living among 

 the mycelial filaments of the Fungus supply them with nutriment, and receive in return that amount of moisture and protection, which 

 enables them to grow and multiply. The hyphae in fact are parasites upon the gonidia, abstracting from them the materials they manu- 

 facture as green plants. But the gonidia, though thus kept in check are not exterminated, and the survivors go on growing and multiplying, 

 so that there is always an excess over and above the wants of the lichen. As the gonidia multiply so do the hyphae, and the scale on 

 which the business is carried on necessarily becomes larger. It is a combination for the supply of continually increasing wants, which 

 almost suggests forethought in its striking adaptation. 



Fig. 1. The so-called Iceland Moss is a Lichen, growing on the ground with its thallus erect It may be procured in the dry 

 state from the chemist, as it is used medicinally. 



The lobes of the thallus are numerous and tufted, and the edges are fringed with short teeth. The so-called 

 "fructification" is rare, and is called an Apothecium, because the surface of the receptacle is very slightly concave 

 (when the receptacle is excessively concave, it is called a Perithecium). 



Pig". 2. The Thallus or flat expansion of the Lichen shows in vertical section several distinct layers : — 

 Cortical or superficial layer of closely applied thick-walled cells. 



Gonidial layer, a looser layer of intermingled hyphae in the meshes of which are entangled green rounded 

 cells — the "gonidia." 



Medullary layer of thread-like cells forming the bulk of the thallus. 

 Cortical layer as above 



Pig. 3. Vertical section of the " Fructification " and underlying Thallus. 



Asci containing Ascospores originating from the colourless filaments here called the Sub-hymenial layer. 

 Paraphyses are barren filaments. 



. Pigs. 4 and 5. The Asci are club-shaped and the Ascospores are elliptical. 



Pig. 6. Spermogonia occur on the margin of the thallus and produce Spermatia. 



Life History. — The Sub-hymenial layer of the Thallus produces numerous Asci, each containing several Ascospores. The Asci 

 absorb moisture as they ripen, causing their membranes to swell, until finally the tension is so great, at the top of the tube, 

 that it gives way and the sudden collapse jerks out the spores. The moist spores are accompanied by hymenial gonidia, 

 and put forth several embryo-tubes, some of which lay hold of the substratum, and the others embrace the gonidia and ramify 

 to form the thallus of the Lichen. 



The Spermogonia are considered to be Male Organs ; and as they appear before the fructification is formed, it is very 

 probable that a Female Organ lies imbedded in the thallus, to which the Spermatia are conveyed by water. In this case the 

 fructification would be developed as a result of fertilisation. 



