WILLISTON: DISCOID GEMMAE IN RADULA 339 
this marginal cell the shoot later appears to arise from the central 
part of the thallus. The second type of procedure is seen in the 
germinating gemmae of R. flaccida. In this species from five to 
eight differentiated peripheral growing points immediately give 
rise to the apical cell of the shoot. The formation of a thallus- 
like body preliminary to this stage seems to be suppressed. The 
growth of the outer peripheral cell and its adjoining cells in R. 
flaccida to form a thallus, corresponds to the growth of the thallus 
in R. Hedingeri after the differentiation of the apical cell of the 
shoot. R. protensa probably belongs to the first category and 
germinates according to the procedure followed by R. Hedingeri. 
BOTANICAL LABORATORY, YALE UNIVERSITY. 
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