SOUTHERN CALIFORNIA FERNS 101 
but Notholaena cretacea would be an excellent subject 
for investigation, for its fronds roll up into very compact 
balls. The same intermittent manner of growth is uni- 
versal among the rock lichens of arid countries, and some 
of the mosses possess it. 
I do not know how often this alternation of active and 
suspended life may be repeated before a frond exhausts 
its ability to respond; nor whether the duration of the 
life of a frond is measured by a definite number of months, 
or by the number of days of active growth, so that in 
one good season a frond might complete the cycle of 
its existence in a single winter, but in a less favorable 
one might require a longer period. Again, the habit 
is possessed by species which differ in several respects, 
and appears to be more fully developed in some than in 
others, but the degree and nature of these differences 
are not known. These are subjects worthy of investiga- 
tion, and information concerning them would add much 
to our knowledge of fern-life. 
I am recalling to your attention the characteristics 
of the arid Californian ferns, not as peculiar to them, 
but as examples of the phenomena shown, as you are 
aware, by ferns of all similar regions; nor is it to ferns 
alone that the question of life under the condition of too 
much sunshine and heat, and too little moisture is 
presented. It confronts the whole desert flora, and every 
plant must find a solution of the problem under penalty 
of death. How they severally and variously do it is 
one of the most interesting studies in plant ecology. 
Although I have dwelt at such length on the arid 
ferns, if I may so call them, it is not to be inferred that 
there are no others in southern California. They are to 
me the most interesting, because they differ in so many 
ways from the ideal fern, and because, since this region 
has an unfortunate deficiency of precipitation and excess 
of heat, it is the arid ferns which most abound in it. 
But as there are places in which there is a sufficiency, 
