^4 THE PLANT WORLD 



The effect of weathering Is, also, an important factor in 

 the appearance of lichens, hardly less so than wear on the 

 plumage of birds. Parmelia caperata loses with wear the 

 outer green cortical layer, and the white medulla is so exposed 

 that white becomes almost the predominating color, the 

 green remaining only in the concavities of the furrowed 

 thallus which have been protected from wear. Another effect 

 of age is noticed in the rock growing examples. The central 

 portions turn to a dark, and dingy green, finally dying out, and 

 leaving but a peripheral border, suggesting Parmelia centri- 

 fuga (L.) Ach. (See Fig. 5.) Within the open interior, 

 and even in the center of the thallus often spring new plants 

 which develop, until a rosette formed of concentric plants is 

 established. (See Fig. 3.) 



Lichens have been termed by Ruskin the "eternal tapes- 

 tris of the hills," the very antipodes of the ephemeral butter- 

 fly — but very little has been done toward determining the 

 actual life time of lichen plants or the rate of growth. The 

 reason for this has been no doubt their very longevity, and 

 the difficulty attending transplantation. During the past year 

 I have measured the growth monthly of two examples of this 

 species, with the following results, though no doubt sub- 

 stratum, the character of its surface, the exposure, and the 

 climate have much to do with the rapidity of growth. Here 

 in Concord, Massachusetts, on a vegetable substratum of 

 average roughness {Pyriis — apple and pear), and in a cli- 

 mate of periodic rainfall, amounting to about forty inches 

 per annum, (maximum, August, 4.12 in.) and with a hu- 

 midity of 82% (maximum. Autumn), two examples, one 

 with a southern and one with a northern exposure, showed the 

 following outline extent and features of growth. (See Fig. 

 6.) It will be seen that the greatest growth was accom- 

 plished during the months of heaviest rainfall, and not only 

 by the plant with the northern exposure, but on the eastern 

 periphery of this plant. We might learn then, that moisture, 

 and cool shade are most favorable to such follaceous lichens. 



