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surface, giving it a scabrous or powdery appearance. From this exposure 
to the air these cellules become distributed, and without any previous 
fertilization multiply the individual. 
_ The growth and habitat of Lichens are various, and much influenced by 
outward conditions. On account of their densely cellular and somewhat 
spongy nature, they are strongly hygrometric. They drink up moisture 
with great rapidity. They flourish in a humid atmosphere free from 
poisonous elements. Lichens are rarely found in a healthy condition in the 
impure atmosphere of towns. They have a marvellous power of adaptation. 
While they are sensitive to favourable conditions of growth, they can also 
preserve their vitality through long periods of heat and drought. They are 
likewise capable of enduring intense cold, for when every other sign of 
vegetation has ceased, the hardy lichen still peeps out upon the explorer 
amid the snows of the Arctic regions, as well as upon the summits of the 
highest mountains of the globe. The period of lichen-growth varies some- 
what in different species, but in a general sense, all are of slow growth. 
They spread most freely at first; after that, they will remain for years 
showing little or no change. Mr. Berkeley watched one foliaceous species 
for twenty-five years, at the end of which period it remained as when he 
first observed it. Some of the hard crust-forms have undoubtedly adorned 
our mountain rocks for hundreds, if not for thousands of years. 
During recent years considerable progress has been made in this branch 
of Botanical Science, not only in the enlargement of our Lichen.Flora by 
the discovery of new species, but also by the establishment of the system 
of study upon a sounder and more philosophical basis. 
In 1851, a Monograph was published by the Rev. W. A. Leighton of 
Shrewsbury, on the ‘‘British Species of Angiocarpus Lichens, Elucidated by 
their Sporidia.’”” Angiocarpus Lichens are those whose fruit are covered or 
enclosed in a perithecium like some ascomycetous fungi. That Monograph 
introduced the principle of using the internal structure of the fruit, and 
especially the sporidia, as a reliable characteristic in the distinction of 
species. The distinguished author had observed the value and use of certain 
forms and markings of some seeds in the study and distinction of flowering 
plants ; and, by a process of analogy, he was led to the conclusion that the 
spores of Lichens possessed some permanent features which would be equally 
valuable in the elucidation of that polymorphous class, Systematic investi- 
gation confirmed that conclusion. Before this application of the spores of 
Lichens was made by Leighton, these germs had long been known to 
_ Lichenologists ; but, on account of their minuteness, the experienced 
manipulation, and the high magnifying power required to properly observe 
them, the real value of their characteristics had never been realized. No 
attempt had then been made, either in this country or elsewhere, to apply 
