1914] GROOM—TRACHEID-CALIBER 291 
of the subsection PArA-cEMBRA, is xerophilous, has narrower spring 
tracheids than any other East Indian species, the width agrees suf- 
ficiently with those of American species, as it is 31.5 wu; that is, it 
would occupy, from the same point of view, a middle position if 
ranged among the American species of ParryA. The other East 
Indian pine, P. excelsa, belonging to section I, is a member of the 
group StRoBus. Its habitat is not xerophilous, and its spring tra- 
- cheids are 38. 5 u in diameter (compared with 41. 5m of P. Stirobus). 
As table II shows, section II of Pinus displays no such simple 
Systematic or ecological relations as does section I. This may be 
due partly to the difficulties mentioned before in appraising the 
habit and habitat of each species, and in part to defective classi- 
fication of this large section. 
The first three species in table II, having the narrowest spring 
tracheids (29. 5-33 m), are clearly xerophilous in habitat. One of 
the two Pacific types, P. ponderosa (29.5 wu), is, according to Mayr, 
the first pine encountered in going west from the prairies. A little 
farther west, while the narrower (moister) valleys include the 
Mmoisture-loving Douglas fir and P. monticola (43 wu), the broader 
(drier) valleys are occupied by prairie or by P. ponderosa. Still 
farther west, on sandy soil, Douglas fir vanishes and P. ponderosa 
1S accompanied by P. Murrayana (34), which also has narrow 
Spring tracheids. Again, according to Mayr, in a more northern 
Pacific region, where during the vegetative season the prevailing 
relative atmospheric humidity is 80-63 per cent, Douglas fir exists; 
but when the humidity sinks to 54 per cent, Douglas fir is replaced 
Y P. ponderosa. Mave ranges Pacific pines found growing in 
hae temperate regions as follows, according to their demands for 
Moisture, beginning with those demanding most moisture: 
Soil moisture: P. Jeffreyi (47 w), P. Lambertiana (45 »), P. ponderosa 
(29.5), P. Coulteri (30.5 n). 
_‘\tmospheric moisture: P. Lambertiana (45 p), P. Jeffreyi (47 w), P. Coul- 
feri (39.54), P. ponderosa (29.5 p). 
_ Thus, in these pines which replace one another from site to 
Site, the two with widest spring tracheids demand more moisture 
than the other two. It is worthy of note that the moisture-loving 
haploxylic P. Lambertiana has narrower spring tracheids than the 
