306 BOTANICAL GAZETTE {APRIL 
lous in distribution, while those with the widest tracheids belong 
to a subtropical or tropical moist climate. Though in general this 
section of Pinus supports the theory given in paragraph 1, there 
are in it certain species in which width of tracheid does not appear 
to correspond with the supply of available water. Such discrep- 
ancies, whether real or only apparent, may be due to one or more 
of the intervening causes mentioned in paragraph 6. 
4. Species of other North American genera of evergreen Conif- 
erae show differences in the width of spring tracheids that may 
possibly be partly due to differences in affinity; as species of the 
same habitat, but belonging to different genera, may differ con- 
siderably in tracheid width, or, on the other hand, may approximate 
to agreement. Some of these genera, namely, Torreya, Chamae- 
cyparis, Sequoia, and Juniperus, support the view that the width 
of the spring tracheid is correlated with available water supply; 
somewhat favoring the view are Cupressus and Picea; indifferent 
in indication are Abies and Larix. 
5. The theory here propounded derives support from measure- 
ments of the width of the spring vessels of American deciduous 
species of Quercus. For narrowness and wideness of spring vessels 
in the main are respectively associated with scantiness and abun- 
dance of water supply. But in the same kind of habitat the decidu- 
ous black oaks would seem to have narrower spring vessels than are 
possessed by the deciduous white oaks. 
6. Though the evidence as a whole strongly favors the theory 
here propounded, much fuller information is necessary before 4 
safe conclusion may be drawn. Hence this inquiry and the sug- 
gestions here given must be regarded as tentative and issued in the 
hope of stimulating inquiry in regard to factors that may interven®, 
such for instance as the following: climate (including evaporation 
power), exact soil water-content, level of water-table, etc., that 
form the environment of the different species of conifers; ls0, 
depth of root, duration of foliage and size of aggregate leaf surface, 
rate of transpiration, width of sap wood, etc., in the different 
species; also, variations within one and the same species in Tes# 
to the features just mentioned, as well as in the width of the spring 
tracheids, in different habitats. 
IMPERIAL COLLEGE 
Lonpon 
