14 THE PHYSIOLOGY OF STOMATA. 



elaborations of the cuticle or adjacent cells in the form of pits, plugs, or other 

 devices which might be regarded as modifying the effect of the stomatal 

 aperture. We are therefore face to face with the relation of the stomatal 

 openings to the regulation of loss of water, unmodified by other anatomical 

 complications. 



The small amount of chlorophyll (or its absence, if this be the case) in the 

 stomata of Verbena, their size and mobility, the relatively small amount of 

 starch present, together with other features, have made this plant most 

 valuable in the pursuit of light upon the internal economy of the stomatal 

 apparatus. 



METEOROLOGY. 



A more exact appreciation of the precise degree of adaptive value of the 

 stomatal mechanism of these plants may be had if we glance for a moment at 

 the conditions under which they live. There are introduced for this purpose 

 the tables of temperatures for the year 1906 and of rainfall for 1905 and 1906. 

 The latter are plotted in fig. 6, together with a curve of normal rainfall 

 (Coville & MacDougal, 1903). 



Jan. Feb. Mar. Apr. May June July Aug. Sept. 



Fig. 6. — Curves of rainfall for 1905 and 1906 at the Desert Botanical Laboratory, together with 

 a curve of normal rainfall, based upon data for 15 years taken at Tucson, Arizona. 



With regard to the temperatures it will be noted that extreme cold is not 

 known on Tumamoc Hill, where the plants studied are to be found. Growth 

 is therefore possible for the vegetation of the region at almost any time of the 

 year if the soil-water is sufficient. Thus, the ocotillo began to develop its 

 foliage during the latter half of December, 1906, and by the end of the year 

 many individuals were conspicuous for their abundant green vestiture. 

 During the same period the verbena recommenced its growth and at the 

 close of the month many plants showed a fine development, though of course, 

 owing to the lower general temperatures, the growth was in neither of these 



