INTRODUCTION. 
XIX 
might be removed, and a little water added, if there ap- 
peared a necessity for it. 
Ferns, mosses, and all kinds of cryptogamous plants, 
seem to spring up spontaneously in these cases ; and the 
surface of the earth speedily becomes clothed not only with 
a beautiful but a highly interesting vegetation. The rais- 
ing of ferns from seed, in the manner hereafter described, 
offers a ready way of ascertaining beyond question the 
value and limits of each species. 
It has often been considered somewhat unaccountable 
that plants should thrive when deprived of air. I believe 
a philosopher would smile at the idea of a vacuum existing 
in a vessel containing abundance of earth, water, and living 
vegetables ; but let us consider the subject without refe- 
rence to any philosophical inquiry. It must, then, be 
understood as an unquestionable fact, that in closing the 
vessel no attempt is ever made to exclude the air which it 
contains, or even by any experiment to diminish its quan- 
tity ; therefore, admitting the property of air to press 
equally in all directions, we must take it for granted that 
there is as much air in the vessel as in an equal space out- 
side the vessel ; and so, the idea that the ferns are living 
without air not being based on fact, requires no refutation. 
The next source of wonder is, that a fern should tln-ive 
deprived of that fresh air, or that change of air, which, in 
a state of nature, it is constantly enjoying. The term 
fresh air, though so continually used, has no very definite 
meaning. If it applies to air that has not been breathed 
by animals, I believe we shall find that animals alone are 
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