2l6 



PHYSIOLOGY 



published an account of various oil-cells in a large series of calcicolous 

 lichens (Fig. 117). 



The occurrence of oil- ( or fat-) cells is not dependent on the presence of any 



particular alga as the gonidium of 

 the lichen. Fiinfstuck' has described 

 the immersed thallus of Opegrapha 

 saxicola as one of those richest in 

 fat-cells. The gonidia belong to the 

 filamentous alga Trentepohlia um- 

 brina and form a comparatively 

 thin layer about i6o/i thick near 

 the upper surface; isolated algal 

 branches may grow down to 350 /i 

 into the rock, while the fungal ele- 

 ments descend to i r5 mm., and 

 though the very lowest hyphae were 

 without oil — as were those imme- 

 diately beneath the gonidia — the 

 interlying filaments, he found, were 

 crowded with oil-cells. Sphaeroid 

 terminal cells were not present. 



Fiinfstiick' has re-examined the 

 thallus of Petractis exanthematica, 

 an almost wholly immersed lichen 

 with a gelatinous gonidium, a species 

 oi Scytonema. The thallus is homoio- 

 merous : the alga forms no special 

 zone, it intermingles with the hy- 

 phae down to the very base of the 

 thallus; the hyphae are extremely 

 slender and at the base they measure 

 only about i/i in width. Oil-cells 

 are abundant in the form of inter- 

 calary cells about 3-5 /x in thickness. Nearer the surface sphaeroid cells 

 are formed on short lateral outgrowths ; they measure 14-16/j, in diameter 

 and occur in groups of 15 to 20. The superficial part of the thallus is a 

 mere film ; the hyphae composing it are slightly stouter and more thickly 

 interwoven. 



Bachmann^ and Lang' have further described the anatomy of endolithic 

 thalli especially with reference to oil-cells,_ and have supplemented the 

 researches of previous workers. New methods of cutting the rock in thin 



^ Funfstiick 1899. ^ Bachmann 1904'. * Lang 1906. 



A B 



Fig. 117. Lecidea immersa Ach. A, sphaeroid 

 fat-cells from about 8 mm. below the surface 

 X 550. B, oil-hyphae in process of emptying : 

 a, sphaeroid cells containing oil ; b, cells with 

 oil-globules x 600 (after Funfstiick). 



