BoT.-Voi.. I.] PEIRCE— NATURE OF LICHENS. 23 1 



this question will be clearer if we use a homely analogy. 

 A cow would never climb to the top of a twenty-story build- 

 ing, but once elevated to this position in opposition to her 

 ordinary habits and to the force of gravitation, would she 

 be any more advantageously placed than her more common- 

 place relatives in barn and pasture ? Though algal cells 

 cannot suspend themselves in mid-air, or live indefinitely 

 exposed to excessive heat and dryness on sun-baked rocks, 

 it does not necessarily follow that they are any better off 

 when associated with fungus hyphse and contributing to the 

 formation of a lichen, than are their more commonplace 

 relatives which are not in the air and have fallen on more 

 advantageous places than exposed rocks. 



VI. The Influence of Mechanical Strains 

 ON Growth. 



As reported in a previous paper on R. reticulata (Peirce, 

 1898), the mere wetting of a thallus or thallus fragment, 

 and the subsequent drying, bring about no material change 

 in the length, breadth or thickness. During the rainy 

 season, however, when the lichen remains wet for consider- 

 able lengths of time, growth will take place while the lichen 

 is expanded and soft, thus permanently increasing the 

 dimensions. The lichen will not contract, when it dries 

 again after the rainy reason has passed, to the dimensions 

 it possessed when the rains began; but how much of this 

 increase in size is due to growth solely, and how much to 

 stretching and the consequent change in position of the 

 hyphce, cannot now be stated. It is obvious that this pen- 

 dant lichen, being more than twice as heavy (Peirce, 1898), 

 as well as much softer, when wet than when dry, will be 

 subjected in its different parts to a stretching force vary- 

 ing in the different parts with the weight of the parts 

 nearer the tips. The parts nearest the branch from which 

 it hangs will be subjected to the greatest stretching force 

 or weight, the tip to the least. By stretching force I mean 



