CHAPTER II 



CONSTITUENTS OF THE LICHEN THALLUS 



I. LICHEN GONIDIA 



The thallus or vegetative body of lichens differs from that of other green 

 plants in the sharp distinction both of form and colour between the assimi- 

 lative cells and the colourless tissues, and in the relative positions these 

 occupy within the thallus: in the greater number of lichen species the green 

 chlorophyll cells are confined to a narrow zone or band some way beneath 

 and parallel with the surface (Fig. i); in a minority of genera they are dis- 

 tributed through the entire thallus (Fig. 2); but in all cases the tissues 



Fig. 1. Physcia aipolia 'Hyh Vertical 

 section of thallus. i/, cortex; ^, algal 

 layer; c; medulla; d, lower cortex. 

 X 100 (partly diagrammatic). 



Fig. 3. Collema nigrescens Ac\i. Vertical 

 section of thallus. a, chains of the 

 alga Nostoc ; b-, fungal filaments, x 600. - 



remain distinct. The green zone can be easily demonstrated in any of the 

 larger lichens by scaling off the outer surface cells, or by making a vertical 

 section through the thallus. The colourless cells penetrate to some extent 

 among the green cells; they also form the whole of the cortical and 

 medullary tissues. 



These two different elements we now know to consist of two distinct 

 organisms, a fungus and an alga. The green algal cells were at one time 

 considered to be reproductive bodies, and were called "gonidia,'' a term still 

 in use though its significance has changed. 



