978 MINNESOTA BOTANICAL STUDIES. 



which are exposed to their flight, must have a certain influence 

 in determining the character of the line between mid-strand and 

 back strand. And, again, the chilling of the air at night by 

 the rapid mid- strand cooling has doubtless its effect, slight 

 though it may be. 



Other physical conditions affecting the back strand, and 

 arising lakeward may scarcely be regarded as having their 

 origin in the mid- strand. An example of such conditions would 

 be the cold wind striking the higher back strand formation 

 owing to its exposure to a large area of water, while in another 

 part of the Jake such a wind would be less cold, having passed 

 over a smaller portion of the lake. As the back strand increases 

 in height this influence will become more important. A portion 

 at least of the differences which have arisen between back strand 

 at Oak point, with its stunted Populus and Pinus divaricata 

 trees, its Rosa, Fraxinus and Cerasus shrubs and back strand at 

 Sandy beach with its tall Populus trees and abundant shrub of 

 Taxus, Amelanchier and Rhus may be attributed to the exposure 

 of the Oak point back strand to continuous colder winds than 

 those that strike against Sandy beach. The height above the 

 lake level of the two back strand formations is approximately 

 the same, in the regions studied. Yet the difference in the 

 temperature of the winds received by the two areas during a 

 season must be considerable, amounting without doubt to sev- 

 eral degrees. And even a few degrees more or less in annual 

 temperature is of great importance in the effect upon plant 

 development and plant distribution, as may be learned from 

 the numerous phaenological observations. Among other con- 

 ditions tending to modify the back strand, and falling within 

 this category may be noted spray- action. This affects the 

 back strand especially whenever there is a heavy surf formed 

 under ordinary stress of wind. So at Oak point, where the 

 surf is unusually strong, even upon the back strand during a 

 storm or soon after the storm has passed one may feel the fine 

 spray drifting inland. If this surf- spray is frequently carried 

 across the mid- strand — the region of its greatest influence — 

 alterations in the vegetable formations of the back strand will 

 result. The lowering of the temperature, the increase of the 

 moisture in the air and in the soil, and the slight diminution of 

 the sunlight must all have their effect. 



Passing in due course, to the biological conditions originat- 

 ing lakeward and capable of modifying the back strand, certain 

 general suggestions may be worth bringing forward. The line 



