510 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Other shrubs associated with Cassandra ai'e species of Vacciniam^ 

 Bctula pumila, Alnus incana, Salix Candida, Rhus venenata, Sarracenia 

 2nirpurea, Drosera rotundi/olia, various Orchids, as species of Calopogon, 

 Pogonia, and Cypripedium, with sedges, as Uriophoruin and Sphagnum. 



Not only the adaptations, but the species themselves, are similar over 

 vast areas ; the conditions are unique, and the flora also. 



As flood-plains may develop either forests or not, so where peat-bogs 

 ■existed there may grow up shrubs and trees, or herbs and grasses may be 

 dominant. In this case Bulrushes encroach on Water-lilies, followed l^y 

 sedges, grasses, and willows. Other plants consist of Viola, Potentilla 

 anserina, Fragaria virginiana, Parnassia carolinicma, Gentiana crvnitay 

 Iris versicolor, Sisyrinchinm angustifolimn, Triglochin maritima, &c. 



Prairies are formed by sedges encroaching upon bulrushes, and grasses 

 upon sedges, as the soil becomes raised more and more. Coarse 

 xerophytic herbs may accompany the grasses, as Silphium, Solidago 

 rigida, species of Jlelilofus, Bapfisia, Eryngiuin, Dodecatheon, Phlox, &c. 

 This explanation of prairies around Chicago must not be applied to the 

 great climatic prairies further west. 



The next group considered are the Upland series, such as Rock hills, 

 at first almost bare of vegetation, but by the decay of rock and accumula- 

 tion of organic matter shrubs begin to appear, such asPrwrns virginiana, 

 Pihus Toxicodendron, BJius typhina, Ptclca trifoliata, and Pyriis coronaria. 



Xerophytic tree vegetation follows, and ultimately a mesophytic one. 



The Clay hill is another instance of special type of vegetation. They 

 are morainic in origin, and always covered with mesophytic forests of 

 Oaks and Hickory as the predominant species. The soil is a composite 

 glacial clay, rich in food salts. 



Lastly are considered the various coastal types, such as the Lake-bluff 

 series, at first almost entirely without vegetation. Then herbs find a 

 place, as Equisetuni hyeviale, species of Aster, Melilotus alba. Then 

 comes a xerophytic thicket of Juniperus communis, J. virginiana, and 

 Salix glaucophylla. The following tree stage includes species of Poplar^ 

 Ostrya virginica, Pinus Strohus, and Oaks. 



Lastly are the Beach-dune-sandhill series. Starting from the 

 embryonic stage of a beach-dune, such plants as can stand shifting sands 

 are Ammophila arundinacca, Salix glaucophylla &c., Prunus pumila, and 

 Populus monilifera. 



Various sub-types of dunes and their inhabitants are described. 

 The most ancient beach-dunes are covered with trees and undershade, con- 

 sisting of Corylus americana, Ceanothus americanus, Salix humilis, 

 Vaccinium j^ennsylvanicum, &c., and species of Oaks predominate. 



The paper concludes with a general summary. 



Similar interchanges to the preceding, in the country about Chicago, 

 are described by Alb. Nilsson as occurring in Sweden {Bot. Not. 1899,. 

 pp. 89-101, 123-135). Mr: H. C. Cowles abridges his work as follows :— 

 " Speaking of cliffs and moors, he finds three types of cliffs, those which 

 are forested either with conifers or deciduous trees, and those without 

 trees. On all clift's the first plants are crustaceous lichens. On the 

 conifer cliffs there follow in succession, fruticoso lichens, herbs, heath- 

 plants, conifers. 



