496 
F. L. PICKETT 
ing protonemal type up to independent secondary prothallia produced 
by outgrowths from old plants to which in some cases they were yet 
attached. That specimens found in the field are A. platyneuron 
cannot be determined with absolute certainty. The absence of other 
ferns in the immediate neighborhood and the abundance of prothallia 
close around old clumps of sporophytes leaves little doubt on this 
question. Careful comparison of plants from the field with those 
of cultures, including measurements of cells, have been made, however, 
and in some cases sporophytes large enough to be recognized as 
juvenile forms of A . platyneuron have made the identification more 
certain. 
Summary of Asplenium platyneuron. — The facts of the growth and 
development of the prothallia of A. platyneuron of ecological impor- 
tance may be summed up as follows : There is a variation of a few weeks 
in the time required for the germination of spores. 
The prothallia in experimental cultures withstand a temperature 
of — 12° C. without injury, and in the field they have survived 
exposure to a temperature of 23° C. and later produced sporophytes. 
The resistance to extreme desiccation, brought about artificially, is 
slightly lower than in the case of Camptosorus rhizophyllus , but the 
resistance to extreme natural drought is much more marked than in 
that fern. 
Mature prothallia are uninjured by exposure to repeated periods 
of three to four weeks of drought as met with in nature. 
Extreme sensitiveness to changes in light intensity leads to the 
production of protonemal branched prothallia, or attenuated out- 
growths, either of which may lead to a vegetative increase of the 
gametophyte and the formation of independent plants. 
The prothallia show marked individual differences in reaction to 
light changes and in the power to survive exposure to conditions of 
extreme desiccation. 
The author wishes to acknowledge his indebtedness to Prof. D. M. 
' Mottier and Dr. F. M. Andrews, of the Botany Department of 
Indiana University, for their kind encouragement and valuable 
direction and help in the present work. 
Botanical Laboratory, 
Indiana University 
