294 BOTANICAL GAZETTE [APRIL 
diploxylic P. Jeffreyi of apparently similar moisture-demanding 
habits. The other two with narrowest tracheids are both obviously 
xerophilous as regards edaphic conditions, and one of them is also 
climatically so in so far as it is a Pacific species. 
As a second group of species may be taken those with narrow 
spring tracheids, between 34 and 37min diameter. All these occupy 
physically or physiologically dry soils, or where the evidence of 
this is least obvious, as in P. Murrayana, the species often accom- 
panies P. ponderosa. Although P. inops (36) occasionally is 
distributed together with two species belonging to the moist sub- 
tropical Atlantic region, namely P. echinata (48 p) and P. Taeda 
(49.5), it is in areas less moist than that region, where P. rigida 
(33 m) is also encountered. 
The next two species, P. arizonica (38 w) and P. chihuahuana 
(38.5 u), mingling on the mountains of dry.Mexico, agree closely in 
diameter of spring tracheids, although the two species belong to 
different systematic subdivision of section II of Pinus. Closely 
agreeing with them is P. pungens (38.5 u), which belongs to the 
same systematic group as P. arizonica and is xerophilous in distri- 
bution on the Pacific Coast. 
The fourth group containing four species, with width of spring 
tracheids between 39 and 4o yn, includes one Atlantic species 
growing in very pervious soil (gravel) and three Pacific species 
growing on soils that we may presume are not pervious gravels, 
for P. Coulteri grows on gravelly loam and P. insignis occupies 
sand and is used to fix sand dunes, being apparently uninjured 
by the salt water flung over its roots by spring tides (Mayr). 
Comparison between P. Coulteri and P. ponderosa, which endures 
greater drought, has been made above. 
The next two species, with the width of spring tracheids between 
43 and 44, differ from all those previously discussed in being 
largely cool temperate in distribution; they are northern forms. 
So far as supply of moisture is concerned, it is difficult to see why 
the cool temperate P. Banksiana (43 mw) should have relatively wide 
tracheids, as it can grow well on dry barren sand or in peat bogs; 
and even P. resinosa (44) for a time can endure dry sand. It 
may be that, just as with a higher temperature of climate, tree 
