296 BOTANICAL GAZETTE [APRIL 
Pacific species (nos. 1 and 7) and two Atlantic species (nos. 3 and 
6), all four of xerophilous habitat; also two Mexican species (nos. 
8 and g) growing in a dry climate and at least sometimes on dry 
soil. The three species with widest spring tracheids include two 
Pacific species (nos. 16 and 20), definitely showing larger demands 
for moisture of soil and air than the other Pacific warm temperate 
species, and one Atlantic species (P. resinosa) that is not edaphically 
xerophilous as are the other Atlantic species in this warm temperate 
group. There is not the sharp contrast between the Pacific and 
Atlantic species that there is in the subtropical group, possibly 
because the Pacific species are not at so great a disadvantage as 
regards supplies of moisture, either because they belong to the 
northern Pacific, which is characteristically moister than the south- 
erm (no. 1 partly, and no. 7), or because they grow on mountains 
where the climate is moist (nos. 12 and 20), while the Atlantic 
species with warm tracheids are definitely in dry soils. 
Cool temperate and alpine species.—Of the two diploxylic species 
ranged under this heading, P. Murrayana, the Pacific species often 
on dry ground and reaching alpine situations, has narrower spring 
tracheids (34 4) than has the other, P. Banksiana (43 m), though 
this latter can grow on very dry soils. 
Summary.—The American species of Pinus having the narrow- 
est spring tracheids are all more or less markedly xerophilous in dis- 
tribution; those with the widest spring tracheids are all subtropical 
and more or less hygrophilous in distribution. The few East Indian 
species show similar characters in this respect. In section I of 
Pinus, so far as the measured American and East Indian species are 
concerned, differences in tracheid width run parallel with distinction 
in systematic affinity, thus suggesting that the evolution of the dif- 
ferent groups of this section has been determined by the available 
water supply. 
Other North American Coniferae 
The succeeding table records the width of the spring tracheids 
as given by PENHALLow, and the distribution of other American 
conifers. 
Table III shows that evidence in favor of the view that 
scantiness and abundance of available water supply are respectively 
