204 ENVIRONMENTAL CONDITIONS. 



The numbers obtained by our study are given in table 3, and the 

 chart formed with these is reproduced as plate 35, the locations of the 

 stations used being denoted by small circles. The equiseasonal lines 

 exhibit the data in a graphic manner. Increments of 30 days in the 

 length of this hot season are represented. 



It is noticeable on plate 35 that our lines assume a generally east- 

 and-west direction to the eastward of the one hundredth meridian of 

 west longitude, and a north-and-south direction in the vicinity of the 

 Pacific ocean. Both mountain systems exhibit a tendency to displace 

 the various climatic belts to the southward, a tendency shown still 

 more markedly by the Pacific Ocean, and the same tendency is 

 exhibited to a relatively slight degree by the Great Lakes. The region 

 thus normally without the hot period here considered embraces almost 

 the entire Pacific coast, from the vicinity of San Luis Obispo to the 

 Canadian boundary; also the northwestern part of Minnesota, the 

 whole of the northern peninsula of Michigan, a small adjacent part of 

 Wisconsin, the northern extremity of the southern peninsula of Michi- 

 gan, and the extremely northern parts of New England. This entire 

 region is generally in high repute for summer resorts. 



(E) LENGTH OF PERIOD OF LOW NORMAL DAILY MEAN TEMPERATURES. 



(TABLE 3, PLATE 36.) 



Just as the length of the hottest period may be supposed to influence 

 the appearance or non-appearance of certain plants, so the mean 

 length of the coldest period of the year may have its effect upon the 

 distribution of the same or other forms. It seems improbable, how- 

 ever, that a normally long period of very cold weather may be essential 

 to the full development of any organism. This coldest season always 

 finds the plant in a dormant condition, and it is hardly possible, on 

 physiological grounds, that extreme cold should be directly advanta- 

 geous to its survival, but it is to be realized that nothing is yet quanti- 

 tatively known in this connection. On the other hand, it is quite clear, 

 in general at least, that cold weather tends to exclude many plant- 

 forms from the vegetation of any region where such weather may 

 occur. Numerous plants that readily survive a single frost are com- 

 pletely annihilated by the occurrence of several days of freezing 

 weather. The question here raised refers mainly to the power of 

 dormant plants to retain life under more or less persistent conditions 

 of temperature below that of frost. 



As a critical normal daily temperature mean we here take 32° F. 

 (0° C.) For each station included in the temperature tables of Bulle- 

 tin R of the U. S. Weather Bureau, it has been determined how many 

 days in the year possess a normal daily mean of 32° F. or below. Owing 

 to the averaging and smoothing process by which the daily normals 

 have been derived, these data exhibit a regular annual march, and all 



