1018 MINNESOTA BOTANICAL STUDIES. 



Creviced rocks. Crevice-formaticn is indec d only a variety of 

 irregular surface contour, but from the cleavage of the rock in 

 straight lines intersecting each other, vegetation rows are 

 often established. The crevices, if of long standing, have com- 

 monly been filled with soil and are occupied by their character- 

 istic plants. A large list of crevice dwelling plants might be 

 prepared from the general catalogue of Lake of the Woods 

 vegetation, Scarcely a plant of the region, except the true 

 aquatics, like Utricularia or Potamogeton is altogether unable to 

 maintain itself in a crevice. Yet, notwithstanding this fact, 

 there is a limited group of omnipresent crevice plants which ap- 

 pears on almost every shore. These may be divided as follows: 



I. Crevice thallophytes. Here are included a number of lich- 

 ens, especially Cladonias, with which the region abounds, 

 mosses and ferns. Polypodium and Dryopteris are the most 

 abundant crevice-fern genera. Often a creviced shore is marked 

 off with intersecting lines of green and white, the green be- 

 ing crevices filled with Polypodium, the white, crevices tenanted 

 by Gladonia. 



II. Crevice herbs. The dominant crevice herbs are doubtless 

 Campanula rotundifolia, Houstonia purpurea., Heuchera cimeri- 

 cana, Agrostis hiemalis, Arenaria stricta, Achillea millefolium, Am- 

 brosia psilostachya, Apocynum androsaemifolium and Vleclcia ane- 

 thiodora. As has been indicated above, many others may be 

 encountered, but these are universal. 



III. Crevice shrubs. The dominant crevice-shrubs are Juni- 

 perus sabina, Juniperus communis, Spiraea salicifolia, Rosa ivood- 

 sii, Symphoricarpos, Rhus, Corylus, Diervilla and Cornus. Cre- 

 vice shrubs need no wider crevices than do the herbs, but what 

 is lost in width must be made up in length or depth. 



IV. Crevice trees. An interesting feature of the plant popu- 

 lation is the ready adaptability of certain trees to live in nar- 

 row crevices. The forms most commonly found in such re- 

 stricted quarters are Pinus strobus, Pinus divaricata, Quercus 

 macrocarpo,, Fraxinus americana and Populus tremuloides. It is 

 remarkable to observe the ease with which pine trees six or 

 eight inches in diameter maintain themselves in a crevice less 

 than a foot wide, where the entire root system must dispose 

 itself in the cramped space afforded it, and yet the pines espe- 

 cially thrive wonderfully under such conditions. Some islands, 

 creviced sparingly, maintain what at a distance seems to be a 

 fairly solid and homogenous forest of pine, yet upon close ex- 

 amination this forest will be discovered to consist entirely of 



