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 (Haploxylon), 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 first step in the evolution of this section of Pinus would appear to have been a division into a more xerophilous type (ancestral Paracembra), and a less xerophilous type (ancestral Cembra), and each of these subsections would appear to have undergone similar division into more or less xerophilous groups, that is, into Parrya and Balfouria, also Eu-cembra and Strobus. The two East Indian species, P. Gerardiana and P. excelsa, structurally accord with this theory. 3. Among American species of Pinus belonging to section II (Diploxylon), those with narrow spring tracheids are more xerophilous 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 discrepancies, 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 Coniferae 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 considerably in tracheid width, or, on the other hand, may approximate to agreement. Some of these genera, namely, Torreya, Chamaecyparis, 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 measurements 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 abundance of water supply. But in the same kind of habitat the deciduous 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 a safe conclusion may be drawn. Hence this inquiry and the suggestions here given must be regarded as tentative and issued in the hope of stimulating inquiry in regard to factors that may intervene, 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; also, 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 regard to the features just mentioned, as well as in the width of the spring tracheids, in different habitats.