366 BOTANICAL GAZETTE [NOVEMBER 
is remarkable, growing in all kinds of soil, on slopes of every angle 
and every direction, through wide variations in soil moisture, 
evaporation, light, and temperature. Its wide geographic and 
altitudinal range is an expression of this plasticity. In favorable 
situations it grows rapidly, with straight trunk and branches 
regularly arranged; in the more exposed places it is reduced in size, 
and commonly gnarled and irregular. Distribution of the pines is 
largely a matter of establishment, since the critical stages are seed 
burial, germination, and the young seedling period. Crevices and 
soil-filled spaces between rocks, usually of small area, afford lodging 
places for the seeds; exposed summits and slopes of fine soil are 
mostly covered with grassland. Small areas of soil deposition may 
allow burial of many seeds, and consequent development of dense 
young stands. Seeds germinate well in the tangled mats of 
Ceanothus Fendleri (see -under Ceanothus association). The first 
few years of the seedling are safely passed only when several favor- 
able seasons are successive (at least in exposed situations), as shown 
by RAMALEY (13, p. 30) for the high mesas near Boulder. 
According as establishment is abundant or very sparse in a given 
station, the growth is closed, giving a true pine forest, or scattered, 
. resulting in the well known open or parklike appearance; this is a 
mixed vegetation of which the pines form only one component. 
They may later dominate the whole area if new pines can germinate 
beneath, but on the whole the closed pine forest is relatively infre- 
quent. Just how important an influence in the foothills fire has 
been, and is, is very difficult to determine; it is said by some 
residents that the whole region just north of Boulder Creek was 
once much more extensively forested than now; but if fire is of 
fairly frequent occurrence in a region, it is an environmental factor 
to be taken into account. Its effect is wholly favorable to the 
grasslands and primitive growths, at the expense of the pines 
(fig. 4). 
In the lower and more southerly parts of the foothills, dry grassland and 
particularly primitive grassland form the ground cover in most areas of 
scattered pines. The spiny shrub Ceanothus Fendleri is also commonly seen 
between the trees. Away from the individual trees, and often even at their 
very bases, the plant cover is mostly not different from its condition where 
