LOCAL DISTRIBUTION OF SPECIES. 65 
at a sufficient depth to be available to all plants, whatever may be the 
type of root-system, but rains of much smaller amounts probably benefit 
to a limited degree, if at all, the perennials with deeply lying roots. 
That different species of cactus are able to absorb water from a pre- 
cipitation of 0.5 inch (12.7 mm.) has been frequently observed. For 
example, on the night of October 17, 1907, 0.54 inch (13.7 mm.) of rain 
was recorded at the Desert Laboratory. ‘Twenty hours afterwards the 
flat stems of Opuntia discata, which previous to the rain were wrinkled 
and shrunken, were observed to be plump and smooth. An instance 
is given by Mrs. Spalding (1905) in her study on the mechanical adjust- 
ment of the sahuaro to varying quantities of stored water, in which after 
a rain of o.5 inch, the stems expanded steadily for three weeks. It has 
been observed that with a rainfall of 0.54 inch (13.7 mm.) moisture from 
it may be detected at a depth of 4 inches (10.16 cm.), and from what 
has been said above regarding the nature of the absorbing root-system 
of Cereus giganteus water at this depth, as well as a much less depth, would 
be available to the cactus. 
A review of the rainfall by days at the Desert Laboratory from May, 
1904, to December, 1907, shows that there have been many such small 
rains. There are short periods of precipitation followed by longer ones 
of dryness in spring and autumn which may not be of sufficient amount 
to support the annual vegetation, and which would directly benefit only 
the plants with a root-system similar to that of Cereus giganteus, so shal- 
low as to be within reach of such slightly penetrating rains. 
Of whatever causes may act to bring about the superficial placing of the 
root-system of Cereus giganteus and other cacti, if a direct response to 
external physical agencies, the two following suggest themselves as being 
worth considering. These are: First, the average depth of the penetra- 
tion of the rains; second, need of proper aeration, although the temper- 
ature relations may also be important here. 
As mentioned in the foregoing, a study of the rainfall at the Desert 
Laboratory indicates that many of the rains do not penetrate to a greater 
depth than that of the absorbing roots of the cacti. But that this is not 
the determining cause is probable from the fact that the heaviest rains 
of the year, those of summer, occur at the season of most active plant 
growth and penetrate the ground to a depth much beyond the cactus 
roots. This conclusion is strengthened by the fact that in regions south 
of Tucson, where fleshy cacti are most abundant, the rains are seasonal 
mainly. How far, on the other hand, the need of proper aeration controls 
the placing of the absorbing root-system is undetermined, but it is not 
unlikely an important factor. 
The behavior of the roots of plants growing in porous pots and in pots 
impervious to the air, as well as the plugging of drains by roots, are familiar 
examples of the direct influence of the need of aeration on growth of roots. 
