26o THE PLANT WORLD. 



the air by extracting COo, the oxygen of which will not answer 

 the purposes of respiration so long as it remains in composition 

 with the carbon, and give back to the air a portion of the oxygen 

 in the free condition. Therefore there is some ground for the 

 old notion that the air is especially pure where lichens are abun- 

 dant. Even travelers in wild regions have sought such places for 

 camps and resting places over night, and certain it is that the 

 lichens are very sensitive to conditions of the atmosphere. They 

 tend to disappear in places where man congregates in large num- 

 bers, and are becoming scarce near our cities and larger towns. 

 This is due partly to the disturbance of substrata, but it is also true 

 that the dust and impurities of the air about cities are in some 

 way unfavorable to the lichens. Doubtless the dust falls the pores 

 of the thallus and interferes with the passage of gases, while 

 some impurities interfere with nutrition. or breathing or both. It 

 is not the intention to give the impression that lichens are the 

 great conservators of atmospheric purity, but rather that they 

 contribute their share of work toward this end and that it would 

 be to our advantage if they could be induced to grow more 

 abundantly in and about cities. 



It is also well known that certain crustose lichens are the first 

 plants to attack rocks, and that they aid greatly in the reduction 

 of rocks to soil. It would seem from superficial observation that 

 some lichens might begin to grow on perfectly firm rock, and, 

 gaining a foothold, reach their full size and produce fruit while the 

 rock is still in a firm and wholly undisintegrated condition. For 

 instance, on the very hard Sioux quartzite in northwestern Iowa 

 and in southwestern Minnesota, the writer has found lichens 

 growing on perfectly smooth surfaces supposed to have been pol- 

 ished by wind before, or shortly after, the close of glacial times. 

 Yet this rock shows to the eye or lens no evidence of disintegra- 

 tion and is, macroscopically, in exactly the same condition under 

 the lichens as elsewhere. But in spite of this, it is not generally 

 supposed that the rhizoids of the lichens ever penetrate perfectly 

 firm rock, but rather that the lichen gains a point of attachment, 

 perhaps in microscopic openings, and then begins to secrete an 

 acid which slowly disintegrates the rock, the rhizoids penetrating 



