MacMillan: shores at lake of the woods. 979 



between back strand and mid-strand is what has previously been 

 termed by the writer a tension line — that is, a line between two 

 general groups of plants striving to move in opposite direc- 

 tions. At such a tension line a reciprocal stress is developed, 

 and the plants of the mid- strand strive to enter the back strand 

 while those of the later area in turn attempt to work out upon 

 the former. Wherever a mid-strand bar is sent up into the 

 back strand, there particularly the mid-strand plants insinuate 

 themselves, but even where no favorable physical conditions 

 have arisen to assist, there is a tendency on the part of the 

 lakeward established group to creep in between the meshes of 

 the landward formation. Thus, from this reciprocal biological 

 strain of one formation against the other an irregular boundary 

 zone is developed, and nowhere is the exact line of demarca- 

 tion altogether clear and distinct. Thus characteristic back 

 strand plants such as Convolvulus sepium frequently work out 

 into the extreme mid- strand and with equal adroitness such 

 plants as Elymus canadensis, Artemisia caudata and Prunus pu- 

 mila creep up upon even the most shaded heights of the back 

 strand. 



A peculiar biological influence that modifies back strand, at 

 certain isolated points on Lake of the Woods not connected with 

 the mainland, is the nesting of the gulls and terns with which 

 the lake abounds. By their deposition of guano, and probably 

 too, by their carrying in of seeds, these birds have at various 

 points on island back strand established conditions favorable 

 to the development of vegetation islands that may mark the 

 approximate spot of the rookery long after the birds have de- 

 serted it. The contribution of nitrogen to the soil makes it 

 more suitable as a substration for nitrophy tes and at such spots 

 an overplus of plants demanding considerable nitrogenous food 

 may be found. 



While by no means all the conditions arising lakeward and 

 tending to modify the back strand have been touched upon, 

 enough have been mentioned to show the character of the 

 problems and the discussion may pass on to the conditions 

 arising landward, by which back strand formations may be 

 modified. As in the case of the conditions arising lakeward 

 these may be grouped as physical and biological. The physi- 

 cal conditions are somewhat numerous and depend upon the 

 exposure, contour, and general character of the back country. 

 Back strand upon which there is a drainage from the inland 

 regions behind, differs markedly from that limiting the border 



