V. THE ROCK RAVINE ASSOCIATIONS 



Well-developed narrow rock ravines with nearly perpendicular sides 

 are so well shaded and the air currents are so reduced that the evaporat- 

 ing power of the air is reduced to a minimum. This permits the develop- 

 ment of associations of liverworts, mesophytic mosses, delicate ferns, 

 and extreme shade plants such as Impatiens and Pilea. Upon ledges 

 along the sides of the canyons and at the foot of the walls as soon as 

 suflScient soil accumulates a herbaceous and shrub vegetation similar 

 to the undergrowth of the climax mesophytic forest (q.v.) makes its 

 appearance. With the widening of the canyon and the accumulation 

 of talus the trees of the same forest appear, while as the floor of the 

 canyon widens the vegetation of the flood plain (q.v.) develops upon it. 

 The upper edges of the ravine have a vegetation similar to that of the 

 river bluff (q.v.). 



In the following lists only the more characteristic species of some 

 small limestone canyons near Sag and Lemont, 111., and of the large 

 sandstone canyons of Starved Rock Park are included. No attempt will 

 be made to list the mosses or leafy liverworts, but the thallose liverworts 

 include: Riccia natans, Marchantia polymorpha^ Reboulia sp., Aneura 

 pinguiSj Pellia epiphylla, Blasia pusilla^ Scapania (nemorosa?), Cepha- 

 lozia sp., and Anthoceros laevis. 



Trees and Shrubs 



Taxus canadensis Marsh., American yew. 

 Asimina triloba Dunal., American papaw. 

 Hydrangea arborescens L., wild hydrangea. 

 Ribes Cynosbati L., gooseberry. 

 Aruncus Sylvester Kosteletzsky, goat's-beard. 



Herbaceous Plants 



Phegopteris polypodioides Fee, beech-fern. 



Asplenium angustifolium Michx., narrow-leaved spleenwort. 



Cystopteris bulbifera (L.) Bernh., bulb-bearing fern. 

 " fragilis (L.) Bernh., fragile fern. 



Cryptogramma Stelleri Prantl, slender cliff-brake. 

 *Pilea pumila (L.) Gray, clearweed. 



Parietaria pennsylvanica Muhl., pellitory. 



Saxifraga pennsylvanica L., swamp saxifrage. 

 *Arabis brachycarpa Britton, rock cress. 

 " laevigata Poir., rock cress. 



Impatiens biflora Walt., spotted touch-me-not. 



Aralia racemosa L., wild spikenard. 



SoUdago latifolia L., broad-leaved goldenrod. 



21 



