ASPECT OF THE VEGETATION ABOUT TUCSON. 75 
opened every evening on many branches, so that a display of pure white 
flowers is offered by any individual for a period of 5 or 6 weeks. 
PLANTS IN THE HUMID MIDSUMMER. 
The course of the temperature rises irregularly during June until, 
in the latter part of this month and early in July, thermometers in the 
shade read as high as 112° F., and the surface layer of the soil warms up 
to a point where a thermograph with a buried bulb gives daily records of 
over 100° F., with but slight cooling at night. Lying quiescent in this 
soil are thousands of seeds of plants which are incapable of germination 
in the moist season of the cooler months, and which can not sprout in a 
soil which contains much less than 15 percent of moisture. Snowy piles 
of cumuli begin to be seen on the mountain summits early in July, and the 
earlier short showers are followed by longer ones which spread out over 
the plain in fantastic patterns, generally giving the greatest rainfall of the 
year during July and August. As soon as the precipitation is sufficient 
to bring the soil-moisture up to the critical point, millions of seedlings 
spring into activity, and forty-eight hours may see the entire face of 
the landscape changed in appearance. 
SUMMER PERENNIALS. 
The great barrel-cacti, which have hitherto remained practically dor- 
mant, now having become thoroughly heated up and supplied with water 
drawn in by the network of roots, which ramify in all directions from the 
bases of theirthick stems, immediately underneath the surface of the soil, 
now begin to open aseries of reddish and lemon-yellow flowers, to be followed 
by the formation of a crown of maturing fruits which stay in place until 
the middle of the following summer. The seeds of the sahuaro, which are 
produced in enormous quantities, are devoured by the birds before being 
freed from the fruits, but of the great number that reach the ground and 
germinate, not one in a million survives and makes the curious globular 
plantlet a few inches in height eventually destined to become a giant cac- 
tus. The seedlings of all the cacti form a favorite food of a large number 
of small animals, being juicy reservoirs of water and containing enough 
other material to lead to their destruction before sufficient armament has 
been formed for their protection. 
Some plants, in the lives of which the supply of moisture is the control- 
ling factor, start up again with the summer rains in a season much warmer 
than that earlier in the year in which they have previously been active. 
A Cassia must be reckoned among these, and its brownish pods opening 
in August make a second liberal sowing of its seeds. A low straggling 
shrub, the dragon’s-blood (locally known as ‘‘sangre engrado,”’ as a corrup- 
tion of sangre de drago), Jatropha, spreads its waxy green leaves amid 
many other plants of a grayer, more xerophytic aspect. The greater 
