ASPECT OF THE VEGETATION ABOUT TUCSON. 71 
densely are they crowded that great purple patches are formed on the 
slopes. Occasionally an individual is found which has lost entirely the 
power of making the characteristic color of the flowers, while others lack 
it only partially, and of these an experimental study has been begun. 
The Mexican poppy, Eschscholtzta mexicana, likewise offers many things 
of interest. Its flowers are light yellow or have a distinct admixture of 
red; its petals show entire margins or are deeply cut; the orange eye at 
the bottom of the corolla cup may be clearly defined and sharp or diffuse; 
but, most striking of all, a number of individuals have been found in 
which the foliage has a paler color than ordinary and the flowers are of a 
clear creamy white, the eye at the bottom being the only color retained, 
and at the same time the margins of the petals are delicately frilled, 
making a most striking deviation from the main type, between which 
numerous intermediates are to be found. 
The more prominent structures by which these annuals are fitted for 
life in the desert are not to be looked for in the shoots or leaves, but in 
the seeds and their powers of endurance. Seeds are ripened and thrown 
on the ground in March and April. The surface layers of the soil reach 
a temperature of over 100° F. during the summer months, the summer 
rains come and soak both the soil and seeds, but still no activity is shown, 
and the experimentalist who attempts to use these plants during the 
summer will find that he might as well have sown so many pebbles in 
his pans. The summer cools into the autumn, and cooler nights come, 
followed by the winter rains of December; then and not until then do 
these refractory seeds begin to show signs of life. Two features are possi- 
bly involved in this delayed germination. One is that the seeds need a 
certain length of time for the carrying out of slow changes toward matur- 
ity, which take place during the so-called resting season and which need 
a period of determinate length not to be shortened. Secondly, it is 
quite possible that in some species the baking summer heats, the moist 
soil, the cool nights of autumn, and the rains are a series of stimuli which 
must follow each other in turn and act for a length of time before the 
seedling emerges from its protecting coats. Favor is lent to this view 
by the fact that in some species germination may be induced earlier by 
simulating the summer heats and the winter coolness by the use of the 
oven and the refrigerator. 
SPINOSE AND SUCCULENT FORMS OF THE DRY FORESUMMER. 
The precipitation shows a decrease to o.go of an inch during February, 
and this, with a further diminution to 0.77 inch during March, coupled 
with the steadily rising temperature, brings to an end the lush and luxuri- 
ant vegetation of the moist winter season. Late in March or early in 
April the xerophilous conditions come to full expression. The stimulus 
of the still increasing temperatures and of the decreasing relative humidity 
