von Schrenk — Dissemination of Usnea barbata. 193 



These figures show that the individual threads are very much 

 weaker when wet than when dry, and at the same time they 

 are much heavier and more elastic. The action of the wind 

 is liable to be most effective in times of storm, when it is also 

 raining, in other words, when the thallus is wet and conse- 

 quently most readily torn. It remained to be seen what wind 

 velocities would suffice to bring about the tearing. 



So far as I know no tests have ever been made to determine 

 experimentally the strength or resisting power of any plant 

 member in a stream of moving air. It is claimed that a hur- 

 ricane may tear the leaf lamina, and we know it breaks 

 branches and timber, but we do not know the actual strength 

 of these organs. This has been due no doubt to an inability 

 to measure the velocities of and pressure exerted by any given 

 wind with any degree of accuracy. A piece of apparatus 

 recently devised * enables one to measure with great exactness 

 the velocity of, and pressure exerted by a moving current of 

 air. Unfortunately for the tests made with Usnea t all the 

 conditions existing during a rain-storm could not be brought 

 about, so the results are somewhat higher than had been 

 expected. Nevertheless they are given, as they bring out 

 some interesting connections between longitudinal strength 

 and the tearing force of the wind. 



A pipe 8 inches in diameter and some 20 ft. long was 

 attached to the exhaust of a fan inclosed in a box, and capa- 

 ble of several thousand revolutions per minute. By throt- 

 tling the engine which turned the fan, the speed of the latter, 

 and, correspondingly, the velocity of the air current produced 

 by it, could be varied at will. Into the pipe a window was cut 

 four feet from the end, and into this the cup for measuring the 

 air current, as well as the lichen, was placed. The dry mass 

 of Usnea barbata was first tried. In all the trials the long 

 threads were tied to a vertical post in the pipe by means of 

 silk threads. Repeated trials showed that wind velocities of 

 77 miles per hour were not sufficient to cause any pieces to fly 

 off. The thallus was then soaked in water 30 minutes to 1 hour 



* Nipher, F. E. Trans. Acad. Scl. St. Louis 8: 1. 1898. 



