Volume 12 Number 1 



The Plant World 



A Magazine of General Botany 

 JANUARY, 1909 



RELATION OF PLANT GROWTH AND VEGETATION 

 FORMS TO CLIMATIC CONDITIONS. 

 By J. J. Thornber. 

 On several occasions in Experiment Station publications 

 the writer has noted the relation between climatic factors in 

 the Southwest, especially those relating to temperature and 

 rainfall, and the vegetation forms and the seasons of growth of 

 the native flora. Perhaps nowhere in our country are condi- 

 tions more favorable to extended observation of this kind than in 

 this region where vegetation is still largely in its virgin stage 

 and the controlling factors so pronounced in their effects upon 

 plant growth. On the mesas and mountain slopes and up to 

 altitudes of 4500 feet or thereabouts, the most important con- 

 sideration so far as growing plants is concerned is that of soil 

 moisture, while at the higher altitudes, namely, 7000 feet and 

 above, the matter of temperature becomes of greater consequence 

 to plants than that of soil water. To state this from an 

 observational standpoint, plant growth of one type or another, 

 i. e., summer or winter growth, obtains at the lower altitudes 

 whenever there is sufficient moisture in the soil, favorable tem- 

 peratures being for the most part constant factors; while at the 

 higher altitudes growth of any considerable extent begins only 

 when the temperature becomes sufficiently warm, moisture in 

 quantity for germination and growth being usually present. 

 Since rainfall in Arizona takes place during the winter and early 

 spring months and again during the summer months*, there 

 being two well-defined precipitation periods, it is quite apparent 

 that at the lower altitudes there are two seasons of plant growth, 

 viz., a winter and spring season and also a summer season, while 

 at the higher altitudes already noted there is only one growing 

 season which from the nature of the case begins near the first 

 of June and ends with autumn. 



* Rainfall in the extreme western and southwestern parts of Arizona is very limited. 



