100 VEGETATION OF A DESERT MOUNTAIN RANGE. 



point at which some particular feature of its physiological activities 

 is met by some particular environmental condition that is preventive 

 or unduly inhibitory to it. The minor fluctuations of climate, which 

 have their minimal and maximal values within periods that are as 

 brief as the normal life of a perennial plant, are registered in the infre- 

 quency of every species as it approaches its distributional limit and 

 in the scattered individuals which lie farthest out from the main area 

 of occurrence. The secular changes of climate which have their maxi- 

 mal and minimal points many centuries apart are registered in slight 

 movements of the limits of species, the marginal region of scattered 

 occurrence being, of course, the first affected by such movements. 



The writer has seen no evidence indicating that competition between 

 plants is at any place in the Santa Catalinas responsible for the limi- 

 tation of any species. There is, of course, competition such as that 

 between seedling pines in heavy stands of 10 to 40 years in age, and 

 such competition as occurs between individuals of the same or different 

 species of herbaceous plants in small areas of moist flood-plain. While 

 competition may thus determine the surviving individuals of a stand 

 of young trees, or may determine the composition of a small community 

 of ephemeral or root perennial plants, it is not responsible for the find- 

 ing of a plant in one habitat rather than in another, and is not respon- 

 sible for the exclusion of a species from an area in which it might find 

 favorable conditions. 



It is only consistent with our knowledge of the diversified physical 

 requirements of plants that there should be such great diversity in 

 the location of the belts of altitude occupied by different species, and 

 it accords with our knowledge of the distribution of plants in general 

 that these belts should be wider in some cases than in others. It is 

 possible, however, to pick out groups of plants the limits of which 

 correspond roughly with the limits of the Desert, Encinal, and Forest 

 types of vegetation respectively. Even the plants of equatorial regions, 

 in which there is a notable constancy of climate, both daily and annual, 

 are able to endure small ranges of climate, or occasionally to endure 

 changes in individual factors which are many times as great as the 

 normal fluctuations of their native climates. In addition to the 

 fluctuations of climate from month to month or from year to year which 

 must be endured by any plant, there are often even greater differences 

 which must be simultaneously endured by the most remotely separated 

 individuals of the same specific stock. For example, Asclepias tuberosa 

 is found from Maine to Minnesota in the north and from Florida to 

 Texas in the south, and thence sporadically in the mountains westward 

 to Arizona. In view of the prodigious range of this somewhat poly- 

 morphous plant it is surprising not to find it reaching a greater elevation 

 than 6,500 feet in the Santa Catalinas. With regard to possible differ- 

 ences in the physiological behavior of the most remotely separated 

 individuals of a plant stock of such wide range, we know little. 



