1. PROTOCOCCUS VIRIDIS Ag. (_Pleurococcus vulgaris_ Menegh., _Cystococcushumicola_ Naeg.). Cells dividing into 2, 4 or 8 daughter-cells and not separating readily; in excessive moisture forming short filaments. The cells contain parietal chloroplasts, and, according to Chodat[281], are without a pyrenoid (Fig. 22). This alga, and allied species, forms the familiar green coating of tree-trunks, walls etc., and, in lichenological literature, are quoted as the gonidia of most of the crustaceous foliose and fruticose lichens. Chodat[281], who has recently made comparative artificial cultures of algae, throws doubt on the identity of many such gonidia. He lays great emphasis on the presence or absence of a pyrenoid in algal cells. West, on the contrary, considers the pyrenoid as an inconstant character. Chodat insists that the gonidia that contain pyrenoids belong to another genus, _Cystococcus_ Chod. (_non_ Naeg.), a pyrenoid-containing alga, which, in addition to multiplying by division of the cells, also forms spores and zoospores when cultivated. He further records the results of his cultures of gonidia, and finds that those taken from closely related lichens, such as different species of _Cladonia_, though they are alike morphologically, yet show constant variations in the culture colonies. These, he holds, are sufficient to indicate difference of race if not of species and he designates the algae, according to the lichen in which they occur, as _Cystococcus Cladoniae pyxidatae_, _C. Cladoniae fimbriatae_, etc. [Illustration: Fig. 23. _Cystococcus Cladoniae pyxidatae_ Chod. from culture × 800 (after Chodat).] [Illustration: Fig. 23 A. A, C, _Chlorella vulgaris_ Beyer. B and C, stages in division × about 800 (after Chodat); E, _Chl. faginea_ Wille × 520 (after Gerneck); F-I _Chl. miniata_; F, vegetable cell; G-I, formation and escape of gonidia × 1000 (after Chodat).] Meanwhile Paulson and Somerville Hastings[282] by their careful research on the growing thallus have thrown considerable light on the identity of the Protococcaceous lichen gonidium. They selected such well-known lichens as _Xanthoria parietina, Cladonia_ spp. and others, which they collected during the spring months, February to April, the period of most active growth. Many of the gonidia, they found, were in a stage of reproduction, that showed a simultaneous rounding off of the gonidium contents into globose bodies varying in number up to 32. Chodat had figured this method of “sporulation” in his cultures of the lichen gonidium both in _Chlorella_ Beij. and in _Cystococcus_ Chod. (Fig. 23). It has now been abundantly proved that this form of increase is of frequent occurrence in the thallus itself. _Chlorella_ has been suggested as probably the alga forming these gonidia and recently West has signified his acquiescence in this view[283].