INTRODUCTION. 



hold dye-agents by the peasantry in many parts of our 

 country ; that in many parts of the world they furnish in- 

 dispensable food not only to cattle but to man ; that they 

 play an important part in the history of Arctic enterprise, 

 inasmuch as they have frequently saved the lives of Arctic 

 travellers; and that they are celebrated in the history of 

 medicine in this and other countries. If, in addition to 

 these high recommendations, we consider that many species 

 have a texture which, by readily imbibing and eagerly re- 

 taining moisture, renders them in a sense independent of all 

 climatal changes, enabling them equally to brave polar cold 

 and tropical heat; that many not only cling with such 

 tenacity as to be inseparable from, but can corrode or dis- 

 integrate, the hardest and barest rocks, even pure quartz ; 

 that the most ample provision has been made by the great 

 Author of all for their reproduction or multiplication, in 

 spite of the most adverse external circumstances, and under 

 conditions fatal to all higher vegetation, both by the mul- 

 tiplicity and abundance of their reproductive cells — which 

 sometimes constitute almost the entire bulk of the plant, — 

 the extremely minute size and delicate nature of these cells, 

 by virtue whereof they are disseminated by every shower or 

 zephyr, and the readiness with which these germinate ; and 



