Fink: Composition of a Desert Lichen Flora 101 



made upon any desert lichens, and similar experiments with some 

 of the lichens about the Desert Laboratory would certainly give 

 some very instructive results. 



Zukal, in summing up regarding the hygroscopicity of lichens, 

 says : " Die Hygroskopicitat ist fiir die Flechten eine hochst wich- 

 tige Eigenschaft, und nicht wenigen Arten ermoglicht sie geradezu 

 die Existenz. Dies gilt besonders fiir die Bewohner jener Gegen- 

 den, wo es nur wenige Tage im Jahre regnet, wie dies z. B. in 

 manschen Landschaften Chiles, Australiens und Nordafrikas der 

 Fall ist."* It is doubtless true that hygroscopicity is of very 

 considerable use to the crustose lichens of the deserts; though 

 the most hygroscopic lichens are not the crustose ones, which 

 seem to constitute almost the whole lichen flora of the desert 

 area under consideration, nor yet the closely foliose ones that 

 form a very small proportion of the lichen flora of Tumamoc 

 Hill, but the loosely foliose and the fruticose species, especially 

 those covered with hairs, cilia and free rhizoids. Experiments 

 similar to those of Spalding and Jumelle, performed upon the 

 crustose lichens of the desert in the driest condition in nature 

 and giving the relation between fresh and dry weight, would give 

 data regarding the amount of moisture retained in lichen thalli 

 during the driest times in the desert. Accompanying this should 

 go observations regarding the length of time that these lichens 

 may be kept dry and then resume active respiration and assimi- 

 lation on the return of favorable conditions. 



Lichens can get water from the surface of the ground or rocks 

 for a short time during and after each rain or wet season ; but 

 they have no special adaptation for storing water like cacti, nor 

 have they organs extending any considerable distance into the soil 

 or into rock crevices by which, like many seed-plants, they could 

 extract soil moisture from any considerable depth. The retreat 

 of the evaporating surface into the soil therefore leaves any 

 lichens growing upon exposed soil entirely in an air-dry environ- 

 ment; consequently, the ability of these lichens, if lichens exist 

 in such habitat, to absorb both water and water vapor from the 

 atmosphere would be of special use to them in withstanding 

 the effects of prolonged drought. This brings us to the ques- 

 * Zukal, H. Op. c. 1346. O 1895. 



