196 Trans. Acad. Sci. of St. Louis. 



This is worthy of note, and attention is called to this fact 

 without attempting any explanation. Of the action of wind 

 upon bodies offering resistance, either wholly or partially, to 

 its passage, little is as yet known. Experimental data are 

 entirely wanting from which one might formulate some law 

 as to how long a wind of given velocity must act to produce 

 a given result (either pressure or tension). Further it is not 

 known what the nature of the longitudinal pull may be. It 

 is suggested that the snapping motion of the filaments, like 

 that of a snapping whip, may account for the efficiency of the 

 lower tension actually found, for it is well known that it is 

 easier to break a long body by snapping it than by direct 

 pull. Further experiments with other plant members are in 

 progress. 



The experiments made with Ramalina reticulata by Peirce 

 and myself indicate that this lichen is much like Usnea bar- 

 hata. Wind velocities of less than fifty miles an hour tore 

 off large pieces of Ramalina. Considering this relatively it 

 would seem that lower wind velocities ought to tear Rama- 

 lina. Not having seen this lichen growing I cannot state 

 this for it as I could for Usnea. A comparison of wind 

 velocities on the Atlantic and Pacific coasts shows much 

 higher average velocities on the Atlantic than on the Pacific. 

 It seems that we have here a good instance of adaptation to 

 their environment on the part of these two plants, represent- 

 ing similar ecologic factors in two widely separated regions. 

 Usnea, growing where higher wind velocities were the rule, has 

 a tensile strength somewhat above that necessary to resist the 

 averasre wind velocities and greater than that of Ramalina, 

 whose habitat is where lower average wind velocities are the 

 rule. It would evidently not be advantageous to have a 

 tensile strength so low that almost every wind would tear the 

 thallus, for then the plant would not grow at all. 



As pieces of the thallus grow very well when separated 

 from the parent plant, this mode of dissemination is a very 

 effective one. It resembles closely that of Tillandsia usne- 

 oides. Just as Tillandsia rarely forms good seed, so the 

 hanging forms of Usnea rarely form apothecia, the vegetative 

 method of propagation having become the prevailing one. 



