478 
F. L. PICKETT 
that under peculiar culture conditions the prothallia of many ferns 
will produce special proliferous buds or gemmae. 
In the present paper there will be shown yet another type of vegeta- 
tive increase, and since the plants under consideration show no ten- 
dency to form special proliferous branches or gemmae, the literature 
dealing with that phase of the subject will be passed with the above 
brief mention. 
(B) It has long been held generally that abundant moisture is 
necessary for the germination of spores and the normal development 
of the prothallia of homosporous ferns. In fact the ability of any such 
structures to live without a considerable constant supply of water has 
but very recently been shown. Goebel in the work referred to above 
(II: 426) describes special tuberous outgrowths produced by the 
prothallium of Anogramme chaerophylla, which covered with soil, may 
survive dry seasons and continue growch upon the return of favorable 
conditions. Little attempt has been made to test the ability of nor- 
mally developed prothallia to survive extended periods of drought. 
Campbell (p. 85) cites a case of prothallia of Gymnogramme triangularis 
which were found growing after the dry season in the neighborhood 
of Stanford, Cal. He also states that the plants of a culture survived 
exposure to dry air in the laboratory during a whole summer. The 
results of the first published attempt to determine the extent to which 
fern prothallia could survive conditions of extreme desiccation ap- 
peared in an article by the present writer in 1913 (Pickett, 1913). 
Discrepancies in the results of that set of experiments, and the possi- 
bility of broadening the scope of the work to include other species 
have led to the present study. 
(C) The influence of light on the germination of fern spores and 
the development of prothallia has been the subject of more investiga- 
tion than both the topics just mentioned. All investigators agree as 
to the necessity of light for normal complete germination, and to the 
tendency of plants to develop attenuated or filamentous forms when 
in reduced light. The present work is concerned primarily with the 
effect of variation in light intensity under otherwise constant condi- 
tions. 
This paper embodies the results of attempts to determine the 
ability of the prothallia of two ferns, grown under control, to survive 
exposure to conditions of extreme desiccation, to extreme drought 
conditions as found in nature, and to extremes of temperature as 
