1914] GROOM—TRACHEID-CALIBER 305 
2. Considering the Atlantic species, and taking separately the 
two series representing respectively the white oaks and black oaks, 
the former of these begins with the two species just referred to, 
continues with two species of variable habitat as regards soil, passes 
to one in which the optimum soil is rather moist but varies, and 
concludes with two species confined to thoroughly moist soil. The 
last of these also occurs in the region where the American pines 
exhibit the greatest caliber of tracheids, namely, Florida and the 
Gulf States. 
The series of Atlantic black oaks commences with wider vessels 
than the series of white oaks. The first species shows no special 
choice of soil, but can grow farther north than any other American 
warm temperate species; the next two species clearly show prefer- 
ence for dry situations, or at least a capacity for thriving on dry 
-Sravels; the series, like that of the Atlantic white oaks, concludes 
with two species confined to thoroughly moist soil on the borders 
of SWamps and rivers. 
Summary 
1. There is considerable evidence that the width of the spring 
tracheids in evergreen Coniferae is largely decided by two factors, 
systematic affinity and available water supply. So far as the latter 
is concerned, the spring tracheids are generally narrowest in species 
of xerophilous habitat. 
2. In American species of Pinus belonging to section I (Haplo- 
xylon), variation in the width of the spring tracheids runs quite 
Parallel with difference of systematic affinity and of available water 
Supply (including influences promoting transpiration). Thus the 
‘ist step in the evolution of this section of Pinus would appear to 
have been a division into a more xerophilous type (ancestral Para- 
CEMBRA), and a less xerophilous type (ancestral CeMBRA), and each 
of these subsections would appear to have undergone similar divi- 
Sion into more or less xerophilous groups, that is, into PARRYA 
and Batrourta, also Ev-cempra and Strosus. The two East 
Indian Species, P. Gerardiana and P. excelsa, structurally accord 
With this theory. : 
3- Among American species of Pinus belonging to section i 
(Diploxylon , those with narrow spring tracheids are more xerophi- 
