PLANT ASSOCIATIONS AND HABITATS. a 
named are entirely capable of successful growth in the full light of the 
sun, provided the root-system is still in soil that supplies a sufficient 
amount of water. The light relation, notwithstanding the apparent choice 
of shade on the part of these species, appears, therefore, to be subordinate 
to water-supply as a factor determining local distribution. 
(b) LIGHT-LOVING SPECIES. 
Many of the winter annuals of Tumamoc Hill grow everywhere in the 
open, where they are fully exposed to the sun, and to a greater or less 
extent, also, where there is some shade; such are species of Harpagonella, 
Pectocarya, Plantago, Daucus, Erodvum, and other genera. ‘These also are 
far more numerously represented on northern than on southern expo- 
sures, but from the fact that the range of light intensity under which 
they habitually accomplish their normal development is so great, there 
can hardly be any doubt that the local distribution of these species, like 
that of the preceding sections, is correlated first of all with water-supply; 
and far less, if indeed to any appreciable degree, with light intensity. 
In its ultimate analysis, therefore, the local distribution of the winter 
annuals here represented is fundamentally based on the water relation. 
So broad a conclusion should receive confirmation from the determination 
of physiological constants involving a prolonged series of experiments, 
but the appearance or non-appearance of both these and the summer 
annuals is so obviously related to rainfall that any other conclusion as 
to their relation to water-supply is, with present evidence, impossible. 
The summer annuals appear after the summer rains, during the period 
of the highest temperature of the year. Once the ground has become 
wet they germinate and develop with great rapidity, some of them grow- 
ing to a large size and in such numbers as to form, in many places, a 
conspicuous feature of the vegetation. They bear the same relation 
to summer rains that the winter annuals do to winter rains, and their 
behavior necessitates the same conclusion as to the fundamental impor- 
tance, in their case as in that of the winter annuals, of the water relation. 
Not less certain, however, is the fact that the habits of both are directly 
correlated with differences of temperature. It is impossible, for example, 
to induce the germination of the seeds of winter annuals in summer tem- 
peratures, unless they have previously been subjected to a low temper- 
ature. Thus while water-supply, with annuals as with perennials, is a 
chief factor in determining distribution in space, differences of temperature 
determine with remarkable precision and certainty the distribution in 
time of the winter and summer annuals, and the face of the landscape 
is twice a year changed to correspond with their biological habits. It 
results that areas which are densely covered with winter annuals in 
February and March afford ample room and all the necessary conditions 
of development to the luxuriantly growing summer annuals of August 
and September. 
