LOCAL DISTRIBUTION OF SPECIES. 43 
which they owe their origin. For this study the gulch located a few rods 
southwest of the Laboratory has been selected, because of its convenience 
of access and because other differences than those resulting from aspect 
are negligible. This gulch has been formed by erosion and is still slowly 
extending its area. By reference to the geological map it is seen that 
the underlying rocks are of the same character on both sides. Whatever 
differences of soil, therefore, are observable at the present time are not 
to be referred to original differences of the rocks, but are due to changes 
incident to its gradual formation, such, for example, as the accumulation 
of vegetable mold on the shady side. 
A comparison of the photographs of the two sides of the gulch Ai points 
directly opposite (plates 9 and 10) shows at a glance some of the more 
striking differences in their plant covering. The sahuaros, which are a 
most conspicuous feature on the right side, facing west of south, are 
hardly seen on the opposite side, and the same is true of Encelza farinosa. 
On the other hand, there are various plants on the shady (left) side which 
are either very meagerly represented on the right or are not found there 
at all. 
For the purpose of more exact floristic comparison of the two slopes, 
six areas of 100 square meters each were located, three on each side of 
the gulch and as nearly opposite as practicable, and a careful enumera- 
tion made of the perennial species of plants growing upon them. Peren- 
nials alone were counted, for the reason that they constitute the more 
permanent vegetation, though the annuals present the same general 
facts of distribution. The use of such marked areas gives a fairer com- 
parison than would a collection at large, though the latter would increase 
the number of recorded species. On these six areas the whole number 
of perennial species found on both sides of the gulch was 48, of which 
24 were found on the right side and 39 on the left. The number of species 
within these areas common to both sides is 15. There were 9 species 
found on the right side which were not found on the left, and 24 on the 
left side not found on the right. 
The most significant fact is that upon these representative areas more 
than two and a half times as many species of perennials were discovered 
growing exclusively on the left bank with its northerly exposure than 
on the right bank of the gulch with its exposure to more severe desert 
conditions. 
Still more impressive is the great difference in number of individual 
plants. As already stated, certain species well represented here show 
no aspect preference; but with many the case is widely different, as is 
shown by the list of 10 characteristic species on the six marked areas. 
(See table 1 on page 44.) 
