44 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
TABLE 1.—Characteristic species on the marked areas. 

No. of indi- No. of indi- 













: viduals. . viduals. 
Species. Species. 
Right.) Left. Right.| Left. 
Encelia farindsa,.. ee eee | 1r2 3 Spheralcea pedata....... 4 a7 
Cereus giganteus sae. 45 eS) O Brickellia-‘coultera.. 22 & « 4 28 
Menodora scabra 2 aa 12 94 Lippia wright see yas O 2 
ADUtUON CHSp Mie ye eee 14 O Brodiza capitata......... O IIO 
A Mcanuniye. ee. ae eee oO 52 Janusia gracilis soe) .&.. > II 86 


A comparison of the annuals, especially as regards numbers of indi- 
viduals, would be still more striking. When the winter annuals are in 
their prime, those on the right bank make no impression on the eye from 
the opposite side, while by reversing one’s position and looking over on to 
the left side from a point near the Laboratory, its whole face appears 
covered with a rank growth of annuals. There is an apparent lack of 
good soil on the right side, but none the less there are many spots where 
there is good soil that is unoccupied, or almost unoccupied, by plants. 
This condition of affairs has evidently come about gradually. Humus 
has slowly accumulated on the left side of the gulch, and the conditions 
have improved from year to year for the growth of plants, but the original 
difference of the two sides is one of aspect, and it is this that is to be 
regarded as the primary cause of the great differences in their vegetation. 
We may, therefore, consider aspect, or direction of slope, from a purely 
physical point of view, as the essential cause from which, as a starting- 
point, the differences of condition and of vegetation already noted have 
taken the form which they exhibit to-day. Originally and fundamen- 
tally, the difference between the two sides of the gulch is simply one of 
insolation, a difference that may be expressed in physical measurements. 
It was thought worth while, therefore, to make a series of temperature 
records for a period of some months. ‘These were begun December 17, 
1906, and brought to an end May 20, 1907. Measurements of light inten- 
sity and readings of wet and dry bulb thermometers were also made 
during a part of this period, and reference will be made to these later. 
For the temperature records four sets of instruments were employed, 
namely, two registered maximum and minimum thermometers for soil 
temperatures; four ordinary soil thermometers; a number of black-bulb 
thermometers, and a set of standard U. S. Weather Bureau thermom- 
eters for air-temperatures, all of which were tested and compared at the 
beginning and during the progress of the work. ‘Two stations were 
established on opposite sides of the gulch at points where extreme con- 
ditions prevail; that on the right side being exposed to full sunlight 
nearly all day, while that on the left was backed by a wall of rocks and 
was well shaded. The maximum and minimum thermometers first men- 
