Width. Length. Width. Length. Width. Length. Width. Length. Width. Length. Width. Length.

TABLE 5.—Changes in dimensions of stomata irrigated with water after remaining some months in absolute alcohol.

TABLE 6.—Measurements of three freshly taken stomata, open in water and closed, partly or nearly, by iodine (Verbena ciliata).

It will be apparent from the foregoing brief description of the two plants chiefly concerned in the pages to follow that they offer decided advantages for a preliminary study of physiology of the stomata in desert plants.

The ocotillo is a highly specialized spinose type, producing normally at least two complements of leaves annually; these are certainly no more specialized anatomically than those of, say, Lactuca scariola. The verbena is a successful herbaceous desert plant with leaves of no more specialized anatomical characters than those of numerous mesophytes, as, e. g., Raphanus. In point of size, the stomata of both plants are approximately of the average size, and their numbers per unit of area are not reduced.

In both plants the stomata are of a simple type and have no accessory

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).

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

plants rapid as compared to that which ensues after the spring and summer rains. During the cool and moist period of the winter season the conditions approximate closely those of a northern cool, moist, though not wet spring, when the ground is wet but the air cool, a high relative humidity, with dews and with an abundance of sunshine. The leaves of the plants which grow at this time are in no danger of wilting, and indeed never show it, and, from the teleological point of view, there is no reason why the stomata should not be wide open. I have, however, not found them open more than six-tenths their possible extent of opening during the warm period of the day, though exceptional stomata show a somewhat wider opening.

TABLE 7.-Rainfall, Tumamoc Hill, Tucson, Arizona, for 1906, in inches.

The approach of the summer is marked by cessation of rain, increase of temperature, and lowering of the relative humidity.* At the same time,

*No accurate data for relative humidity for a sustained period are available, so that general statements must suffice. With reference to the possible high humidities in a given area in the absence of rain see Cannon, 1906. The problem of the desert in this regard is one which commends itself to study. Many areas have a distinctly desert character, but are favored by high atmospheric humidities, and these are contiguous in many places to the desert areas having the opposite condition, e. g., Southern California and Arizona (McGee, 1906).

the soil-water is depleted (Livingston, 1906), though sufficient may be retained to carry some forms of vegetation through to the summer rainy period. Verbena ciliata is in this respect remarkably hardy and, from its anatomical character, quite unexpectedly so. The height of the dry season is one which is trying in the extreme, one would think, and still, with a supply of water TABLE 8.—Rainfall, Tumamoc Hill, Tucson, Arizona, for 1905, in inches.

TABLE 9.—Temperatures for the year 1906, Tumamoc Hill (°F.).

Rainfall.

0.465 2.24

0.048 4.48

0.90

23.183

TABLE 9.-Temperatures for the year 1905, Tumimoc Hill (°F.)—Continued.

not obviously great, the foliage of the ocotillo and of Verbena will continue. normal for a fairly extended period. When the soil-water is sufficient there is no evidence in the behavior of the stomata that even the highest temperatures and strongest insolation affect the plants unfavorably. As I shall show, when the conditions are so trying as to bring about wilting, the stomata then close; otherwise they open to their full extent during certain portions of the day, maximum dimensions having been observed day after day during the hottest weather. And, moreover, as will appear, the differences of temperatures are not without relation to the physiological processes evident within the guard-cells, although these are not in the direction of reducing the transpiration rate when, aside from the condition of wilting, the danger from transpiration would appear to be greatest.