259 
Groom.—Remarks on the Oecology of Coniferae. 
family as regards high latitude: it would be interesting to know if farther 
north the altitudinal limit is higher than that of other Magnoliaceae. The 
magnoliaceous Zygogynum also has wood of the same type, according to 
Van Tieghem (’ 00 ), but particulars concerning the exact habitat and 
evergreen or deciduous nature of the species seem to be lacking. The 
Trochodendraceae are composed of five genera. Of these Trochcdendron 
and Tetracentron possess wood that is devoid of tracheae; while Cercidi- 
phyllnm , Euptelea , and Eucommia have true tracheae. Accordingly, we 
find that Trochodendron is evergreen: whereas Cercidiphyllum , Euptelea , 
and Eucommia are deciduous. Again, the evergreen Trochodendroii with 
its tracheidal wood marks an extreme of distribution in the family as 
it occurs in the alpine region of Japan. Tetracentron sinensis is, however, 
deciduous, with leaves of hygrophytic structure. Yet its tracheides are much 
wider than those of Trochodendron^ and at their dovetailing ends have very 
numerous pits with pit-membranes so extremely thin that the arrangement 
is the nearest approach to vessels with lattice-work perforations. It is 
interesting to note that the Trochodendraceae have their centre of preserva¬ 
tion in China and Japan, where so many evergreen Conifers have been 
preserved: while Drimys Winteri , an extreme southern form, grows side 
by side with evergreen species of Fagus , austral Conifers, Myrtaceae, and 
Proteaceae, also species of Berberis and Ilex. 
Degeneration of tracheal xylem to tracheidal xylem has certainly 
taken place in certain xerophytic species of Tillandsia ( T. usneoides and 
others). 
That this tracheidal structure of the evergreen north-temperate 
Coniferae is not a mere inevitable correlation phenomenon, but constitutes 
a mechanism that is best fitted to the evergreen Coniferae, is suggested also 
by the wide distribution and the survival of the Coniferae: and this ques¬ 
tion will be discussed later in the paper. Here I may merely indicate some 
possible advantages reaped by such temperate evergreen species through 
this tracheidal structure of the wood. It is easy to conceive that the 
acquisition of wider vessels would be a source of danger by facilitating 
a flow of water to the buds at times when available water was plentiful, and 
thus inciting the plant to produce a larger surface of foliage, and that 
of a less xerophytic type ; such a change would bring with it the risk 
of injury or death from desiccation during either the physiologically dry 
or wet season. Again, we know that the rate of transpiration of a plant is 
greatly increased by increase in the rate of absorption and consequently of 
supply of water; it may therefore be that the narrowness of the tracheides 
(or tracheae) is itself a device for opposing resistance to the ascent of water, 
and for thus depressing transpiration. Strasburger (’ 91 ) demonstrated 
that more pressure is required to force water longitudinally through 
coniferous wood than through dicotylous wood possessing fair-sized vessels. 
