508 JOCIiXAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Thalictrum, Trillium, Mitella, Dicentra, and Sanguinaria, with many 

 mosses and liverworts. 



Such ravine conditions are more favourable for plants than any others. 



Rock ravines exclude landslides, hence the sides are often vertical. 

 Denudation is slow, and therefore more stable conditions exist. Being 

 shady and often dripping, rockbound gorges abound with cryptogams, as 

 ferns, mosses, &c. Shade-loving flowering plants occur, as Lnjxi t iens a,nd 

 Pilea. As the canon broadens out and the slopes become less steep, 

 shrubs and trees appear, though a typical mesophytic forest is rarely seen. 

 Whether it be sandstone or limestone, the vegetation is essentially alike : 

 showing that is the physiographic stage of a region which is of the 

 greatest importance ; so that rock, as such, or e\en the soil which comes 

 from it, is of less importance than the aerial conditions and exposure in 

 determining vegetation. As the valley deepens and widens, there appear 

 two phases, the river-bluff' and the bottom. The exposure to wind, sun- 

 light, and changes of temperature increase ; moisture decreases. The 

 liverworts and moisture-loving mosses disappear, while a " xerophytic "" 

 undergrowth now flourishes. Antennaria, Poa comprcssa, Equisetum 

 hyemale, and Polytrichum are common types. Commencing at the top 

 they spread downwards, often almost to the water's edge. The shrubs 

 consist of such as Ptclea trifoliata, Cclastrus scandens, Bhus typhina 

 and B. glabra, Prunus virginiana, Physocarpus opulifolius, Pyrus 

 coronaria, Amelanchicr canadensis, and Ostrya rirginica. The last of 

 the mesophytes to die are trees such as Tilias and Acers ; but they 

 cannot be succeeded by their own kind, inasmuch as the critical seedling 

 stages cannot be passed successfully. 



In the rock-ravines, when passing into xerophytic stations, conifers such 

 as Pinus Strobus and Thuya occidentalis find their home. The herbs 

 and undershriibs of a xerophilous nature consist of such plants as Scla- 

 ginclla rupcstris, Campanula rotundifolia, Pellcea atropurpurea, Talinum 

 tcretifolium, Opuntia Bafincsquii, &c. 



The next physical feature is the formation of a flood-plain. When 

 river-slopes become more and more gentle, mesophytes spread upwards 

 over them. A true aquatic flora is now characteristic of the meandering 

 stream, which could find no foothold in the earlier and more rapid stages 

 of the river, which, moreover, may be only present after storms. A brook- 

 side flora now appears as Symplocarpus fcetidus, Asclepias incarnata^ 

 Chelonc glabra, Polygonum sagittatum, species of Eupatorium, Lobelia, 

 Mentha, Lycopus, Bidens, and Alnus incana. 



When streams are old enough, and therefore slow enough to support a 

 pond vegetation, they have become essentially depositing rather than 

 eroding streams. The order of appearance is the giant Ragweed {Ambrosia 

 trifida), Willows, River Maple {Acer dasycarpum), the Cottonwood 

 (Populus monilifera), and the Ash (Fraxinus americana). 



Gradually the growing flood-plain becomes dry enough to permit the 

 germination and development of a true mesophytic flora. The preceding^ 

 trees are then replaced by Elms {Ulmus americana and/«/ra), the Bast- 

 wood {Tilia americana), Walnut {Juglans nigra) and Butter-nut (/► 

 cinerea), and the Pig-nut {Carya porcina). 



There are many lianes climbing over the trees, e.g. the Greenbriar 



