xxvi INTRODUCTION 



the cortex have been induced by the action of the alga on the fungus. 

 The larger part of the thallus might therefore be regarded as equivalent to 

 a gall: "it is a cecidium, an algal cecidium, a generalized biomorphogenesis." 

 The morphological characters of lichens are of exceptional interest, con- 

 ditioned as they are by the interaction of the two symbionts, and new 

 structures have been evolved by the fungus which provides the general 

 tissue system. Lichens are plants of physiological symbiotic origin, and that 

 aspect of their life-history has been steadily kept in view in this work. There 

 are many new requirements which have had to be met by the lichen hyphae, 

 and the differences between them and the true fungal hyphae have been 

 considered, as these are manifested in the internal economy of the com- 

 pound plant, and in its reaction to external influences such as light, heat, 

 moisture, etc. 



The pioneers of botanical science were of necessity occupied almost 

 exclusively with collecting and describing plants. As the number of known 

 lichens gradually accumulated, affinities were recognized and more or less 

 successful efforts were made to tabulate them in classes, orders, etc. It was 

 a marvellous power of observation that enabled the early workers to arrange 

 the first schemes of classification. Increasing knowledge aided by improved 

 microscopes has necessitated changes, but the old fundamental "genus" 

 Lichen is practically equivalent to the Class Lichenes. 



The study of lichens has been a slow and gradual process, with a con- 

 tinual conflict of opinion as to the meaning of these puzzling plants their 

 structure, reproduction, manner of subsistence and classification as well as 

 their relation to other plants. It has been found desirable to treat these 

 different subjects from a historical aspect, as only thus can a true under- 

 standing be gained, or a true judgment formed as to the present condition 

 of the science. It is the story of the evolution of lichenology as well as of 

 lichens that has yielded so much of interest and importance. 



The lichenologist may claim several advantages in the study of his 

 subject : the abundant material almost everywhere to hand in country 

 districts, the ease with which the plants are preserved, and, not least, the 

 interest excited by the changes and variations induced by growth conditions; 

 there are a whole series of problems and puzzles barely touched on as yet 

 that are waiting to be solved. 



In field work, it is important to note accurately and carefully the nature 

 of the substratum as well as the locality. Crustaceous species should be 

 gathered if possible along with part of the wood or rock to which they are 

 attached ; if they are scraped off", the pieces may be reassembled on gummed 

 paper, but that is less satisfactory. The larger forms are more easily secured; 



