122 NINTH REPORT. 



Passing northward, the characteristic vegetation found to prevail through- 

 out the upper part of the ravine and the occasional tributary ravines, con- 

 sist of Juglans cinera, J. nigra, Hicoria glabra, H. ovata, H. minima, Tilia 

 americana, Quercus alba, Q. velutina, Q. rubra, Q. Alexandrii, Ulraus amer- 

 icana, U. fulva, U. racemosa, Acer saccharum, A. rubrum, A. nigrum, Popu- 

 lus deltoides, Fagus americana, together with Hamamelis virginiana. Sas- 

 safras sassafras, Carpinus caroliana, Ostrya virginiana, Prunus serotina, 

 Opul aster opulifolia, Cornus florida, Benzoin benzoin, Corylus americana, 

 and others. A conspicuous feature is the large number of saplings of red 

 oak (Quercus rubra), maple (Acer saccharum, A- nigrum, A. rubrum), and 

 beech (Fagus americana). As undergrowth, some of the early flowering 

 forms are foimd. The character of the humus-flora is also noteworthy. 

 Hepatica hepatica, Arisff'ma triphyllum, Asarum canadense, Actsea alba, 

 are very common; relativeh' frecpient is Monotropa uniflora. AVhere the 

 physical nature of the slopes makes rapid weathering and erosion impossible 

 the almost vertical clay bluffs, dripping with moisture, have principally 

 mosses and liverworts, while Collinsonia canadensis, Cicuta maculata, Im- 

 ]3atiens fulva, Viola and other moistm'e-preferring plants are found at the 

 base. 



The beds of marl occurring here, covered to some extent by a layer of humus 

 have an herbal vegetation differing but little from other ];)laces around them. 

 Of the ferns, Dryopteris acrostichoides is quite common, but mosses, such 

 as Climacium, Hypnum and Mnium are more abundant. The water de- 

 rived from springs near by, and from seepage, dissolves the almost pure lime- 

 stone beds very rapidly. Considerable sections are thus undermined, pro- 

 ducing instability and consequent settling of the beds, and leading finally 

 to the formation of tributary ravines. The calcium carbonate is carried 

 off to the river, but partly met with again in the form of incrustations men- 

 tioned above. 



As has been stated, the present paper is preliminary to a more extended 

 study of field work in physiology; hence more complete data will be included 

 in another report. But even at this stage, the results obtained make it very 

 obvious that the simplest condition in the life-history of these plant asso- 

 ciations is the outcome of a chain of factors. Where no doubt exists con- 

 cerning the more dominant ecological factor, in concrete cases no one of them 

 can be cited as exclusively determining the character of the local flora. 

 Many differences in kind and arrangement of vegetation are results of the 

 united action of the various ecological factors working in concert, which, 

 in turn reacted upon by the condition of the plants themselves, thus give 

 rise to changes in habitat and consequent distribution of plants not to be 

 attributed either to a changing topography alone, or ground- water level, 

 character of soil, etc. Fully to understand these plant groups we must 

 contemplate them as the result of an ever-changing process (as described 

 and defined above on ]3age — ). And if we wish to compare records, the reason 

 should be that we would know how the many active factors are in^•olved and 

 related in the process that is going on. 



University of Michigan, Botanical Laboratory, April, 1905. 



