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500 RELATION BETWEEN THE ALGA AND FUNGUS OF A LICHEN. 
EXPLANATION OF THE PLATES. 
PLATE 2l. 
Phot. 1. Cladonia digitata, Transverse section of a squamule of the thallus showing gonidia 
in various stages of development. 
n. Nucleus, surrounded by the chloroplast. On the circumference of the latter is a 
darkly stained body, probably the centrosome. ‘The cell-wall is indicated by 
a faint line against which are the cut ends of the hyphe. 
dg. Group of daughter gonidia. x 1000. 
Phot. 2. Section as in the above. Twin gonidia with common cell-wall. Note the retieula- 
tions of the cytoplasm. The cell-wall is indicated as in Phot. 1. x 2500. 
Phot. 3. Cladonia pyaidata. Transverse section of a basal squamule showing part of the 
gonidial layer with pyriform ends of the hyph:e closely attached to gonidia; 
they are the extra-cellular haustoria. x 1000. 
PLATE 22. 
Phot. Cladonia digitata. Transverse section of a squamule of the thallus showing a 
159; portion of the gonidial layer next the medulla. The photographs exhibit four 
5&4, planes of the same section of different depths. On the left is a mature gonidium 
in a plexus of hyphæ, on the right a group of daughter gonidia being surrounded 
by hyphæ. x 1000. 
