THE VOICE OF THE DESERT 



160 



itill be here millions of years after we are gone, and 

 much that is here now would never have been here at all 

 if they had not first colonized the inhospitable rocks. 



At the summits of Arizona mountains several different 

 species flom'ish but the most conspicuous, perhaps, is that 

 which appears as a golden yellow blotch hugging the stone 

 so tightly that from a few feet away it might well be 

 mistaken for a vein of mineral. And even at close range it 

 seems more part of the rock itself than of anything grow- 

 ing upon it. 



Like all lichens, it is, oddly enough, not one plant but 

 two, though that surprising fact was only rather recently 

 discovered. All lichens seem so bafflingly featureless that 

 Linnaeus, the first great classifier, did not know what to do 

 with them and ended by putting them into a large pigeon- 

 hole with a lot of other odds and ends. The penetration 

 of their secret had to wait for the perfection of the micro- 

 scope. And what a strange secret it turned out to be! 



No species of lichen is, strictly speaking, a species at all. 

 In every case what we call a lichen turns out to be a fun- 

 gus — something like the mould on bread — and an alga — 

 something like the green threadhke scum on a pond — liv- 

 ing together in close, mutually beneficial association. When 

 the time comes for reproduction, natLire sees to it that the 

 partners shall start life again together. In many' cases 

 what happens is that a bit of the fungus and a bit of the 

 alga break off together. The wind blows the dry fragment 

 away and when it comes to rest in some suitable place the 

 two organisms begin again to grow in partnership. 



Many different species of fungi and of algae have been 

 able to strike up this odd relationship and each different 

 pair constitutes a different sort of lichen. Sometimes the 



