34 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
Another class of instances that may perhaps be put with the phenomena 
of dynamic physiography are presented by the slopes covered with loose, 
sliding tuff fragments about the three quarries, and notably on the south 
side of the west end of Sentinel Ridge. Only Larrea is able to maintain 
a more or less successful foothold, and even this occurs in distinct rows 
or beds running up and down hill, unable to resist the products of erosion 
sliding down the paths between. There is present on such slopes plenty 
of good-sized rock-fragments, as well as soil. ‘The aspect and gradient 
are the most favorable ones to be found, but the sahuaro in the case of 
the west end of Sentinel Ridge is almost altogether absent. Apparently 
the only thing lacking is a sufficiently stable surface, though moisture 
and other physical conditions may also be changed profoundly by this 
instability of surface. 
Associated with the optimum physical habitat for the sahuaro are often 
found certain recurring organic associations. ‘These are a rather unusual 
presence of species of Abutilon, Janusia, Lycium, and several others, on 
warm southerly aspects, in company with sahuaro groups. One of the 
best indicators of such an optimum habitat is Andropogon contortus. It 
almost certainly requires a high degree both of warmth and moistute. 
Hilarva cenchroides has been located on an area on the south side of Tu- 
mamoc Hill in company with a large group of sahuaros. A rare colony 
of Prosopis has been found on the same spot with a typical terrace group. 
Unusually large specimens of Parkinsonia mucrophylla, growing close 
together, are very commonly found associated with good sahuaro groups, 
occupying the same soil. Phoradendron californicum, on the whole 
rather infrequent, may with great certainty be found suspended, often 
in large thrifty pendents, from well-nourished specimens of Parkinsonia, 
standing among thrifty columns of the giant cactus. Not infrequently, 
where these occur at all, young giants may be looked for with success 
under the protection of both mistletoe and palo verde. 
The fact is to be emphasized that rocks, of whatever kind, as found 
on the Laboratory domain, are almost inseparably linked with the pres- 
ence of the giant cactus. These may carry with them the ultimate factors 
governing local distribution in the form of certain soil properties more 
or less constantly associated with the rocks, or the rocks themselves may 
act as water-conductors and water-reservoirs by means of the air-space 
enveloping them in dry times. The moisture of spaces between adjoining 
rocks, especially basaltic, may be heated by their presence, thus giving 
a temperature favorable for germination, as is suggested by the occurrence 
of the smallest sahuaros found growing in such places. The absence 
of the species appears to be caused by rapid erosion and instability of 
surface in some places, in others a prominent réle may be played by the 
impervious caliche, while in still others the most favorable conditions 
of rock and soil can not make up for the absence of a sufficient degree 
of insolation, or, what is self-evident, for the lack of seed. 
