TH1-: AMERICAN BOTANIST 105 



and M>uth trend i>t' the mountain ranij^cs allowed our plants 

 to elude the ice ^heet and to migrate hack into territory form- 

 erl)- inhahited after the ice sheet had retreated. 



It not intre((uently happens that one species is limited in 

 it> distribution h\- its dei)endance upon another si)ecies. Fungi 

 and such parasitic flowering plants as the dodders and some 

 orchids cannot, of course, he distributed farther than the 

 range of their host [)lants. The chestnut blight must come 

 to an end as soon as all the chestnuts and chinciuapins are des- 

 troyed unless it learns to live upon some other species in the 

 meantime. Even more curious are those ranges that are 

 modified bv insects which p(.llinate the blossotns and thus en- 

 sure a Continuance of the stock. The yucca and the yucca 

 moth, for instance, are so closely interdependent that one can- 

 not long survive in a locality or spread into new regions, with- 

 out the other. It thus happens that the draining of a swamp 

 or the cutting down of a woodland in one part of the country 

 may destroy the feeding grounds of some insect that aids in 

 maintaining a very different species in some other area. 



It is only in recent years that we have come to realize 

 tile imi)ortant jjart the soil pla\s in the distribution of plants, 

 but we now know that vegetation can he divided into two 

 great groups depending upon w'hether they are tolerant or in- 

 tolerant of acid in the soil. Most plants, especially the c<Mn- 

 nion plants of our gardens, do not thrixe in a soil that is acid, 

 but manv others absolutely refuse to grow in any other. 

 The rhododendron, azalea, huckleberry and trailing arbutus 

 belong to this group, not to mention various water ferns, 

 violets, and bog-plants generally. It is apparent, therefore, 

 that it is not mere accident that determines where a plant shall 

 grow. Each species does the best it can under the circum- 



