MacMUlan : shores at lake of the woods. 977 



Of biological conditions, which seem to modify the back 

 strand when compared with the niid-strand, the more important 

 are those which originate landward, but there are some which 

 originate in the back strand formation. The influence of 

 shade increases most rapidly in proximity to the areas of great- 

 est moisture, and a competition for light which is somewhat 

 more vigorous than over the drier areas arises among plants 

 established in the favorable areas for robust growth. Hence a 

 segregation of plants into groups may take place, and the 

 struggle between these groups becomes an important factor in 

 the final distribution over the whole formation. Therefore in 

 the back strand, rather than in the mid- strand, one finds such 

 partially dependent plants as Celastrus, Parthenocissus, Poly- 

 gonum cilinode and Convolvulus sepium supporting their weak 

 stems upon the stronger shoots of other plants. 



Considering next the second group of general conditions un 

 der which the back strand is modified, one may note those con- 

 ditions which arise lakeward. Of this group the chemical 

 modifications are not so important as the physical and biolog- 

 ical. The physical conditions are of two sorts, those physical 

 modifying conditions originating in the mid-strand, and those 

 originating outside the mid strand zone. The biological condi- 

 tions, however, originate primarily in the mid-strand. 



An important physical modification arising primarily in the 

 mid-strand but affecting the back strand is deposition of sand 

 blown landward by the winds or moved landward by the surf 

 or ice until it becomes a portion of the back strand area. Such 

 encroachment of sand has a tendency to reduce the back-strand 

 group of plants to a level with the mid-strand group. And 

 since this sand is more readily carried along certain paths than 

 others, owing to less resistance of established vegetation or 

 because of favorable surface contour of the back strand, there 

 is a clearly marked isolation of back strand masses of vegeta- 

 tion between successive mid strand strips which have pushed 

 landward. Evidently the breadth of the mid-strand area will 

 be an important factor in this process. If the mid-strand is 

 narrow the process will not be undertaken upon a large scale 

 but with a wide mid- strand belt these infiltrating arms or rib- 

 bons, perpendicular to the general mid strand formation are by 

 no means unusual or inconspicuous. The edge of the back 

 strand next the mid- strand is affected also in other ways by the 

 lake formation; for example the reflected glare of the 

 sunshine and the impact-force of fine sand-particles upon plants 



