1905] CANNON—WATER-CONDUCTING SYSTEMS 407 
the phenomena noted. It seems, however, on the surface of things, 
that such explanation must take into consideration the differences 
in the length and the character of the growing season of the two 
classes of plants. The irrigated plants probably do not cease growing 
during any month of the year and are most active when the tem- 
perature is also the highest. 
The curve of the growth of rae {| 
these plants probably is | | | 
very nearly parallel to that Anes 
of the temperature for the | 
year. The non-irrigated Pa | 
plants are subject to the | 
peculiar climatic conditions | 
of the desert. The summer | 
| 
| 
is the season of the maxi- 
mum temperature. At 
\ 
Tucson there are each 
year two rainy seasons: a L\ | 
that of winter, January- ae Bad | | 
February; and that of sum- ee) bd il | L L 
mer, July-Au gust. The Fic. 9.—Mean yearly rainfall and maxi- 
period of the greatest vege- mum temperature at bs segeuibobiars Metin 
tative activity once ar the temperature (rem =10°F.); solid line is rainfall 
i ‘ in inches (1° =o.2'). 
time of the midsummer 
rains (it is not known how the winter rains affect the growth 
of these forms), and all growth ceases very soon after the 
rains are over. The curve of growth of the non-irrigated plants 
would be very similar, no doubt, to that for the yearly rainfall. 
Fig. 9 expresses ina graphic manner the annual rainfall and tempera- 
ture at Tucson. It may be taken also to indicate the rate of growth 
of the non-irrigated and the irrigated desert plants. ; 
Whatever may be the effect in detail, it must be admitted, I think, 
that the growth of the non-irrigated plants, and this may also be true 
of their transpiration, is directly associated with the distribution of 
the annual rains. Further, the intensity of this growth, as sipeesabai 
by the structure of these plants and the general vegetal conditions 
of the desert, is probably directly connected with the fact that the 
