1020 MINNESOTA BOTANICAL STUDIES. 



I. Theloschistes formation. Enormous patches of these 

 bright orange or red lichens may be seen upon the cliffs of the 

 Lake of the Woods district. They are best developed on Shoal 

 lake, on Crow Rock island, in Whitefish bay and upon the high 

 rocks of Yellow-girl bay. Such cliffs acquire a highly colored 

 appearance and seem to offer conditions more favorable for the 

 development of Theloschistes lychneus and the others than do 

 rocks lying at a smaller angle with the horizon. 



II. Umbilicaria formation. Upon some precipitous cliffs 

 vast numbers of Umbilicaria lichen-thalli are prominent. Such 

 seem more copiously developed upon the irruptive rocks than 

 upon the quartzytes and gneisses of the region. 



III. Polypodium formation. Precipitous cliffs green with 

 Polypodium are not uncommon. In this case there are small 

 crevices, or at least an irregular surface to the rock. 



IV. Campanula formation. When the surface of the cliff is 

 seamed with small crevices Campanula gains a foot-hold, and 

 often develops almost to the exclusion of other plants. 



V. Juniperus formation. Upon some vertical cliffs with 

 numerous larger crevices an almost solid formation of Juni- 

 p>erus communis has been observed. 



In general it is apparently a characteristic of the precipitous 

 cliff that its plant population is homogeneous. Where upon a 

 flat rock surface of the same area a dozen or more species 

 would be established, upon the precipitous cliff only half as 

 many may be found. This fact illustrates the influence of rapid 

 drainage, constant unilateral illumination, or general shade, 

 and reflection or radiation of heat upon the plant inhabitants. 

 A smaller number are able to endure the conditions of the per- 

 pendicular rock than can flourish upon the fiat rock. 



HUMUS SHORES. 



Certain types of humus shores, have already been considered. 

 Morasses are regarded as a special type of such shores upon 

 which a distinct zonal distribution is readily established. Mud 

 flats may also be regarded as humus. Both of these, however, 

 have been redistributed, to some extent, by the action of the 

 water. It remains to note the occurrence of ordinary humus 

 in place, along some sheltered shores. Conditions favorable 

 for its development are as follows: the bank must be gently 

 sloping, secluded from the wind, heavily wooded and thus 

 shaded from the direct rays of the sun, undisturbed by running 

 water, and of such general even contour that the soil-water 



