288 BOTANICAL GAZETTE [APRIL 
are likewise wanting. To select one example. By several botan- 
ists Pinus Jeffreyi is described as growing on “dry” or ‘‘very dry” 
gravelly slopes. This statement aroused my doubt because the 
caliber of the tracheids, according to my hypothesis, seemed to 
speak in an opposite sense. On referring to H. Mayr’s (3) work 
I found that he states that this gravel must contain a rich supply 
of water, though the water must not be stagnant. 
5. Information as to depth of root is wanting. The lack of this 
information is especially significant in relation to American species 
of Pinus, which show so strong a tendency for growth on sandy 
soil. A species described as growing on dry sand may, by virtue 
of its long roots or a high water-table, be growing in wet sand. 
6. Other features besides width of spring tracheid may affect the 
supply or expenditure of water, as thickness of sap wood, percent- 
age of spring wood, size and structure of leaf, and aggregate surface 
(including duration) of the leaves of the species. 
Despite the possibilities of disturbance by these factors, the 
evidence clearly points to the view that caliber of spring tracheids 
of different species varies directly with climatic or available edaphic 
humidity, and inversely as the conditions tending to induce desic- 
cation. It also varies with the systematic position of the genus oF 
species. 
As I was working especially at the structure of the wood of 
Pinus and as this genus offers the largest number of species and the 
widest range of habitats of any American coniferous genus, the first 
inquiry concerns it. 
Pinus 
For the purpose of this discussion, the genus Pinus will be 
divided into two sections. Section I includes the species whose 
leaves are haploxylic, whose dwarf shoots have a more or less de- 
ciduous sheath, and whose wood has non-denticulate ray tracheids 
and bordered pits on the tangential walls of the outer summer 
tracheids. Section II includes diploxylic species with a persistent 
sheath, denticulate ray tracheids, and no universal tangential 
pitting in the outer summer tracheids. In the succeeding tables 
the mean diameter of the spring tracheid is the mean between the 
radial and tangential diameters. 
