38 Allan Hancock Pacific Expeditions vol. 26 



described by Harvey from England. The apparent wide range of the 

 species calls for a more complete study of its variability and ecology. 



Working Key to the North Pacific Species of Pleonosporium 



1. Thalli partially endophytic in Codium .... P. codicolum sp. nov. 



I. Thalli not endophytic 2 



2. Thalli corticated, at least below 3 



2. Thalli without significant cortication 6 



3. Thalli distichously branched essentially in one plane through- 

 out 4 



3. Ramuli of higher orders polystichous .... P. polycarpum Gardner 



4. Plants under 2 cm. high 



P. pygmaeum (but see P. dasyoides) 



4. Plants 4-15 cm. high 5 



5. Polysporangia sessile; plants lightly corticated below 



P. complanatum Tayl. 



5. Polysporangia pedicellate; cortication usually heavy except near 



branch tips P. dasyoides (P. pygmaeum 



appears to be a dwarfish, little-corticated variant of this) 



6. Thalli radially branched P. globuliferum Levring 



6. Thalli distichously branched 7 



7. Cells of main axes only about as long as wide 



P. pinnatum Okamura & Segawa 



7. Cells of main axes mostly 1.5 to 4 diameters long 8 



8. Determinate branches from principal axes usually simple .... 9 

 8. Determinate branches from principal axes usually with 1 or 



more orders of branchlets 11 



9. Plants 5 mm. high or less; polysporangia with about 12 spores 



P. pusillum Yamada 



9. Plants 2.5-5 cm. high; polysporangia with about 32 spores 10 



10. Lower parts loosely clothed with rhizoids; polysporangia 

 grouped, not seriate, borne on short branchlets in special 



fertile areas P. rhizoideum sp. nov. 



10. Lower parts not especially clothed with rhizoides; poly- 

 sporangia seriate, sessile and adaxial on ordinary deter- 

 minate lateral branchlets P. mexicana sp. nov. 



II. Lateral branches usually bearing an abaxial branchlet from 



the basalmost cell 12 



11. Lateral branches usually with lowermost cell lacking a 

 branchlet and the first branchlet arising adaxially from the 



second cell 14 



12. Plants ± densely branched in 4-5 orders; polysporangia 



and spermatangia unknown P. abysicola Gardner 



12. Plants ± openly branched in mostly 3 orders 13 



