256 Groom.—Remarks on the Oecology of Coniferae. 
younger age of the irrigated specimens might be responsible for the smaller 
calibre of the vessels. 
It is thus possible that the absolute number of vessels in any correspond¬ 
ing zone of growth in thickness (in an annual ring, if present) is greater in 
the case of the irrigated plants, and that even the width of the large 
tracheae is greater. 
But another consideration arises in connexion with Cannon’s results, 
namely, that since comparison was made between stems of equal thickness 
but unequal age, we do not know which stem bore a larger leaf-surface, nor 
which shoot had a larger transpiration. 
It will be interesting if more critical comparison of the two kinds of 
individuals does show what Cannon’s results at present apparently suggest, 
namely, that when these xerophytic individuals modify their foliage in 
a mesophytic direction, there is a tendency for a reduction in the number 
and calibre of the tracheae. 
Even then, however, we should not be justified in concluding that the 
change of structure in the individual under changed circumstances marks 
out the line of evolution of the species under like prolonged changes of 
environment. 
Thus the consideration of desert plants so far gives us no safe guide as 
to the structure of wood in relation to xerophytic foliage. And the question 
now arises as to whether there is any relation between the structure of the 
wood of a temperate species and the evergreen habit. 
Examining the three tall British woody evergreen species—box, holly, 
and ivy—which seem to be oecologically allied to evergreen north- 
temperate conifers, we find that two of the three are characterized by the 
small calibre of their tracheae. 
Buxus sempervirens has tracheae that are no greater in calibre than are 
the spring-tracheides of the Scots pine. Ilex Aquifolium has vessels that 
are scarcely wider. In Hedera Helix , the tracheae are neither scanty nor 
minute ; though they are narrower than those of Bryonia and Clematis they 
are broader than those of Lonicera Periclymenum. Eliminating the climb¬ 
ing plant, the two others, like a number of cold-temperate Coniferae, thrive 
best in damp situations, and show the nearest possible tracheal approach to 
the tracheidal wood of Coniferae. In fact, all the evergreen trees of cold- 
temperate Europe, whether coniferous or dicotylous, have approximately 
the same xylem mechanism in their older stems. As the smallness of the 
number of these dicotylous trees prevents us from drawing the conclusion 
that the evergreen habit is linked with the narrowness of the tracheae, it 
seemed possible that an examination of one genus with deciduous and 
evergreen species might throw light on the question. 
Accordingly, I measured the maximum size of the vessels in American 
deciduous and evergreen species of Querciis. The specimens used were 
