28 Transaclions BriiisJi Mycolo^ical Society. 



the relations are naturally somewhat different. The fila- 

 mentous forms are represented in one genus by Cladophora 

 in others bv Trentepohlia. The latter alga has frequently 

 thick corrugated walls. In the lichen Coenogonmm, which 

 contains Trentepohlia, the composite structure is filamentous; 

 the fungus either travels alongside the algal strands or 

 surrounds them by a hyphal network. Growth is apical and 

 considerable development in length may be attained ; the 

 fruits which — as in all lichens are fungal fruits — 

 arise laterally on the strands which are thus vitally active 

 well back from the growing point, the alga as well as the 

 fungus maintaining all along a healthy existence. In other 

 genera, such as in (jraphideae, the Trentepohlia filaments 

 are broken up and they lose their distinctive form and colour, 

 but if they reach the open they at once resume their normal 

 filamentous growth. 



In Graphideace indeed the s^mbionts are mutually de- 

 pendent to a striking degree : they lead but a meagre exist- 

 ence until contact or even mere association is established, 

 when each partner is seen to take on new vigour. There 

 has been no record of parasitized Trentepohlia cells. 



The great majoritv of our familiar lichens are associated 

 with Protococcus or closelv allied species, the algae that 

 in damp conditions cover stones, palings and trees with 

 a cheerful green. The region of growth in lichens is at 

 the edge of the thallus and there, as the hyphse extend and 

 branch, active cell-division goes on in the algal cells. These, 

 as a general rule, are herded into a narrow zone beneath the 

 upper cortex, where conditions are most favourable for photo- 

 synthesis. A section through this zone shews an abundance 

 of green cells, some in the stage of increasing. Existence may 

 be, doubtless is, restricted, but does not seem to be 

 particularly unhealthy and interchange of food-stuffs must 

 be constant between the symbionts. It has, however, been 

 observed once and again by various workers that certain 

 algal cells may be disturbed if not destroyed by the fungus. 



A projection or haustorium from a fungus hypha may 

 pierce the outer wall of the alga ; the attack frequently excites 

 the alga to prompt cell division, which by this means becomes 

 free from the haustorium. In other instances, however, the 

 haustorium penetrates to the centre of the cell, branches 

 inside and destroys the contents. Elenkin, a Russian 

 Botanist, claims to have proved by his researches and 

 experiments, that such destruction of algal cells goes on to 

 a very large extent at the lower side of the algal zone. He 

 found in that region so many empty and destroyed algal 



