CHAPTER VII.—PHENOMENA OF VEGETATION.—LICHENS. 397 
hand the same species of Alga may ‘serve as host to different species of Fungi, and 
serve accordingly as a component part in very different forms of thallus. 
The following list contains the genera and groups of Algae which are known 
to form Lichens; the reader is at the same time referred to Schwendener’s and 
Bornet’s special works and to some others also which will be named below, and to 
the general works on the Algae’. Figures 167-169 represent some of these Algae 
as they appear in the thallus of the Lichen and in most cases in connection with the 
hyphae of the Fungus by which they have been attacked. 

FIG. 167. Lichen-forming Algae. The Alga is in all cases indicated by the letter g, the assailing hyphae by 4, A Protococcus 
viridis, Ag. (Cystococcus, Nag.) attacked by the germ-tube from a spore of Physcra parictina. B Scytonema from the thallus of 
Stereocaulon ramulosum. C Nostoc from the thallus of Physma chi , Mass, D Ge from the thallus of 
Synalissa Symphorea, Myl. E Protococcus sp. (Cystococcus) from the thallus of C/adonia Jurcata, P. ACDE magn. 950, 
B 6so times. After Bornet. 

1. Chlorophyll-green Algae. 
(a) With short usually round cells living free or loosely united, but not forming 
filaments or cell-surfaces, the group of ‘ Palmellaceae’: Protococcus, Kg., Pleuro- 
coccus, Menegh., Cystococcus, Nag. (Figs. 167 A, Z, 168), Dactylococcus, Nag., 
Stichococcus, Nag. 
(6) With cells firmly connected together into filaments and cell-surfaces, the 
group of ‘Confervae’: possibly species of Ulothrix and especially the Chroolepus- 
forms (Trentepohlia, Mart., Bornet) (Fig. 169) distinguished by the orange colour 
of the contents (haematochrome) in addition to the chlorophyll. To these may 
be added the genus Phyllactidium said by Bornet to be the Alga of Opegrapha 

! See especially Nägeli, Gattungen einzelliger Algen ; also Kirchner, Algenflora von Schlesien. 
