XXXll 
INTRODUCTION. 
elastic ring, by a series of jerks, throws out the seed : three 
of these capsules are represented in the centre of the figure 
on the title-page. These capsules are also called thecce 
and sporangia : they are generally seated on the back of 
the frond in clusters, often called sori ; but I have carefully 
avoided the use of these words, thinking the English ones 
— i. e. capsules and clusters — better in every respect ; they 
are more correct, more expressive, and more intelligible. 
The clusters of capsules are frequently hidden by a thin 
membranous cover, which I have called an involucre ; it is 
often called an indusium : the white space in the title-page, 
in which the three capsules are placed, shows the shape of 
one of these involucres. 
And now I believe I have only to offer my best thanks 
to those botanists who have rendered me such zealous and 
important assistance during the progress of my work ; and 
I trust that my coadjutors will not think me guilty of un- 
kindness or ingratitude in substituting this general acknow- 
ledgment for individual thanks. Were I to make out 
a list of all the correspondents who have assisted me it 
would be wearisome from its length, and I cannot consent 
to make a selection of particular names when the object of 
all has been the same : as a matter of course the commu- 
nications of some correspondents have been more available 
than those of others, yet the same spirit of kindness and 
goodwill has influenced all, and all therefore have an 
equal claim on my sincere and heartfelt gratitude. I have 
endeavoured to exercise the greatest care in acknowledging 
every communication in its proper place. 
