232 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



in this connection not the forcible expansions which follow 

 the wetting and consequent swellings of a dry lichen, but 

 the actual stretching strain which the distal parts exert by 

 their own weight upon the basal upper parts, a strain which 

 increases with the wetness and with the actual increase in 

 substance. Once thoroughly wet, the lichen will not absorb 

 still more water, but it may grow. We must distinguish, 

 then, between growth which, in this connection at least, is 

 a purely physiological process, and stretching, which is a 

 purely mechanical one. The growth involves an increase 

 in the size of the cells composing the lichen, is made possi- 

 ble by accumulated foods elaborated by the gonidia and 

 hyph^e, and is very quickly followed if not accompanied 

 by an actual increase in the weight as well as in the volume 

 of substance either itself living or at least elaborated solely 

 by the living substance. 



The pull exerted upon itself by the wet and heavy lichen 

 is parallel with the long axis of the thallus ; it is stretched 

 lengthwise, while growth takes place in breadth and thick- 

 ness as well as in length. Hegler (1893) has shown for 

 higher plants that a pull exercises two distinct influences; 

 the mechanical one of stretching, the physiological one of 

 irritating — a pull entirely insufficient for the former being 

 in many cases quite sufficient for the latter. The irritation 

 produced by pulling retards growth in the direction of the 

 pull, stimulating it in other directions both as to the individ- 

 ual cells and also as to the tissues and organs as a whole. 

 A pull so light that it does not stretch a stem or branch will 

 lower the rate of elongation, raise the rate and degree of 

 thickening of the walls, especially of those cells which con- 

 tribute most to the mechanical strength of the plant, and in 

 proportion to the actual increase in length there is more 

 than the usual increase in diameter, by thickening the walls 

 of already existing cells as well as by inducing the forma- 

 tion of new cells. 



Without having made any measurements — a matter 

 requiring infinite patience on account of the low rate of 



