150 Bulh tin Wisconsin Natural History Society. [Vol. 8, No. 3. 



adapted a plant species may be for growth in a particular locality, 

 it will not be found there, unless its spores, seeds or vegetative 

 brood bodies at some time were brought to the place. 



The third is a combination of the two factors just mentioned. 

 Organic beings, by their presence, cause certain changes in their 

 physical surroundings, sometimes of a very considerable character. 

 On land at least, such changes are due to plant much more than to 

 animal life, and especially the most complex of vegetative forma- 

 tions, the forest, is the cause of very far-reaching alterations. In 

 a former paper of this series 4 it was suggested that the progress 

 from oak to bass wood-maple-beech forests was due to the gradual 

 accumulation of humus in the former, preparing the soil for the 

 latter, which afterwards gradually drives out the oak by their 

 greater shade tolerance. Similar processes may be looked for in 

 the history of the Lake Shore flora. 



D. The Beach. 



Dr. Cowles, in his paper on "The Ecological Relations of the 

 A egetation on the Sand Dunes of Lake Michigan (Botanical 

 Gazette, vol. XXVII, 1899) divides the beach into lower, middle 

 and upper, applying the former name to that portion constantly 

 washed by ordinary summer waves ; the middle, to that portion 

 exposed to wave action during storms only; and the upper to that 

 above danger of submersion. This classification is serviceable and 

 based on real distinctions. 



The lower beach, here as in Chicago, is practically devoid of 

 plant growth. Even the larger boulders of the shingle beaches, 

 where one might expect some lithophilous algae, are usually de- 

 void of such. On the sand beach, however, is found an occasional 

 specimen of Cakile edentula. This most successful of beach plants 

 is enabled to sprout in such places by sending a strong tap root 

 vertically down for more than three inches before it develops any 

 foliage leaves. I have found such seedlings in the cotyledon stage 

 as late as July 20. but whether such venturesome individuals ever 

 reach fructification, I do not know. As a matter of fact, this part 

 of the beach is in some respects less unstable than the higher 

 portion. The sand, being always wet, packs and is not subject to 

 constant shifting, like the drier zone above. On the pebble beaches, 



4) January 1902, page 17 et seq. 



