BOT— Vol. I.] PEIRCE— NATURE OF LICHENS. 233 



growth and because of the many sources of error to be 

 eliminated — and merely from a study of the structure of 

 this lichen (Peirce, 1898), I think we may justly infer in 

 the light of the foregoing discussion, that the increase in 

 length of R. reticulata is due in greater measure to that 

 stretching of the thallus, by its own weight, which results 

 in bringing the hyphae from their diverse directions into a 

 course nearly parallel with the long axis of the thallus 

 (i. e., parallel with the pull) than to actual growth. The 

 physiological process of growth in length, made less neces- 

 sary if not retarded by the mechanical pull to which this 

 lichen is constantly subject — a pull which is greater when 

 the lichen is softest and weakest — is accompanied by 

 growth in breadth and in thickness and by an increase, by 

 thickening walls, in the mechanical strength of the hyphae. 

 These processes are at least not hindered and, judging from 

 Hegler's work on higher plants, are likely to be stimulated 

 by the longitudinal pull. As a result of the growth in thick- 

 ness and breadth, the normal proportions of the thallus will 

 be maintained and, furthermore, more hyphae will be 

 formed which, running at various angles to the long axis, 

 may by the pull be drawn into courses parallel with it. 



But Hegler has shown one other thing interesting in this 

 connection, namely, that changes in the pull also affect the 

 rate of growth, and that if the pull remains constant for a 

 time the plant will recover its normal rate of growth in 

 length, growing in diameter and increasing in mechanical 

 strength in proportion to the force to which it is subjected. 

 The decreasing weight of our lichen on drying, the gradual 

 diminution of the pulling force, will stimulate it to some 

 growth in length ; but because the lichen stiffens as it dries, 

 it cannot grow much at this time, and hence one can con- 

 ceive of little more growth taking place than will suffice to 

 fix the hyphae in the direction into which they have been 

 pulled. Since the pull does not remain constant in nature, 

 winds and varying amounts of moisture forbidding this, the 

 lichen cannot become accustomed to any one amount of 



