166 ROBEET PAULSON ON THE 



fig. 1). In the fruticose thallus the first three of these are ar- 

 ranged in concentric circles. 



The upper cortex consists of specially modified hyphae. They 

 have very thick walls and are cemented together, forming a false 

 tissue. In some species the upper cortex is continuous, in others 

 it is broken, so that the next or gonidial layer is exposed. 



The gonidial layer consists of the cells of an alga interspersed 

 within the meshes of hyphae ; some of them are perfectly free, 

 others are firmly attached, and again some are completely sur- 

 rounded by a plexus of hyphae ; but the hyphal threads difEer 

 from those of the upper cortex, for the cell wall is comparatively 

 thin, and the extremities of the branches swell out into a pyri- 

 form shape, and thus afl^ord a larger surface for contact with 

 the algal cells. 



The algal cell (PI. 4, fig. 2) when fully developed is spherical, 

 except when subject to pressure from other cells during active 

 growth ; it has a diameter of from 10 to 15 microns. It contains 

 a single bright-green chloroplast which lines the whole of the 

 cell wall ; the nucleus is large and central, and there is on the 

 circumference of the chloroplast a small body surrounded by a 

 light area. The cytoplasm is finely reticulated. This may 

 account for the fine network of hyphae which is said to some- 

 times surround the chloroplast after entering the cell (2) (4). 

 For a thorough examination of such an algal cell it is necessary 

 to employ a 2-mm. oil-immersion objective. The central body 

 has often been referred to by other authors as a p5n:enoid, but 

 repeated examination after various methods of staining led to a 

 decision in favour of the nucleus. The body resisted the stain 

 when heated nearly to dryness on the slide with acid-fuchsin 

 followed by immersion in a warm concentrated solution of 

 potassium bichromate. 



There is much lack of precision in papers and textbooks as to 

 the name of the alga that is common to so many lichens. It is 

 often described as Protococcus, Pleurococcus, Cystococcus. 

 Pleurococcus of most authors was proved by WiUe, 1913, who had 

 examined the original material of Agardh in the Museum of 

 Lund, to be no other than Protococcus Ag. so that Pleurococcus 

 cannot now be used. Cystococcus Nag. is now Chlorococcum, as 



