o2 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. - 
inevitably plays a large part, and it is hard to say how much is to be attrib- 
uted to this and how much to the degree of insolation. 
A third evident feature is the great increase in general density on the 
middle slopes of Tumamoc Hill, that is, the zone immediately below the 
talus belt; and it shows a decided preference for the finer soil that fills 
the interstices between rocks. This is darker, as a rule, has a smaller 
proportion of calcareous matter, is more retentive of water, and the caliche 
proper lies deeper, or is less in evidence. How much this manifest pref- 
erence is due to the presence of soil of this character and how much to the 
presence of rocks is uncertain, since the two almost always go together. 
The sahuaro differs in the evenness of its distribution locally, often 
forming groups where the topography and substratum are favorable. 
The following may be distinguished on the Laboratory domain: 
(1) Cliff groups occur on the sheer basaltic cliffs where aspect is right 
and are usually composed of smaller plants. Conditions for reproduction 
are good in many rock crevices, but, evidently owing to lack of soil, few 
plants reach any size. These plants are above the degree of steepness 
at which maximum density occurs. 
(2) Platy basalt groups occur where this variety of basalt is exposed. 
The most notable example is just under the southern brow of Tumamoc 
Hill, where a group of 22 is founded on this almost laminated basalt. A 
number of the smaller plants are giving way, indicating that reproduc- 
tive conditions are better than those for maintenance. 
(3) Scorvaceous basalt groups.—A few of these are found just north of 
the first quarry. In one 5 plants stand shoulder to shoulder, covering 
a tiny island of this basalt, while a very few scattering ones occur on the 
adjoining practically rockless shoulder. 
(4) Other rock outcrop groups.—Other kinds of outcrop give rise to 
sahuaro groups. Many good examples are seen on the south side of 
Sentinel Hill and the islands west of the wash. In fact, the sahuaro grows 
on all the kinds of basalt distinguished, as well as on the tuff. ‘The latter 
is well seen on the south side of Sentinel Hill. , 
(5) Talus groups occur about the lower edge of the talus, and are very 
significant as regards moisture supply. Borders of talus slopes are com- 
monly seen in the mountains supporting mesophytic species in the midst 
of a hot, southerly slope. They include here: 
(a) Terrace, or upper coarse talus groups: These are found mainly on 
the south side of the Laboratory domain, which is here the real sahuaro 
country. The groups occur usually on the side of the terrace, affording 
greater steepness and a rockier root-bed than the surrounding area. 
Usually the rock consists of bowlders embedded in the fine soil; some- 
times bed-rock terraces occur, as in the region of the copper-bearing 
basalt. 
