26 o 
Groom.—Remarks on the Oecology of Coniferae. 
Finally, the narrowing of the tracheae involves a relative increase in the 
strength of the wood, and a consequent opportunity for economy of 
material. 
(d) Rationale of the Xerophytism of the Coniferae. 
As already indicated, Schimper’s theory, so far as water supply and 
water expenditure are concerned, explains the survival in north-temperate 
regions of the Coniferae which live in places where there is a physiologically 
dry season. 
But, in seeking for a full explanation of the evolution and survival 
of Coniferae, it appears to me that one fundamental factor in their archi¬ 
tecture has been overlooked, namely, the aggregate leaf-surface. 
The amount of water emitted by a plant varies directly with the 
aggregate leaf-surface and the histological structure of the individual leaf. 
Now, if we imagine two plants, living within identical surroundings, to have 
equal powers of absorbing water, but one to have twice the leaf-surface 
of the other, then, if the one having the smaller leaf-surface just manages 
to retain sufficient water for its existence, it is obvious that the other 
will require more marked xerophytic structure in its foliage. Similarly, 
if there be two plants of equal leaf-surface, but unequal powers of absorbing 
water, and the one with the larger power of absorbing water contrives just 
to obtain and retain sufficient water, then the other will again have need of 
a more xerophytic type of leaf-structure. Such a type of xerophytism as 
is not evoked by edaphic or climatic agencies, but is dependent upon the 
organization of the plant itself, may be termed architectural xerophytism. 
And in any terrestrial community of plants, whether it be in an English 
meadow or a sub-tropical desert, one is apt to find a mixture of plants 
showing various grades of architectural xerophytism, mesophytism, and 
tropophytism ; for instance, in a desert one can find growing side by side 
a deep-rooted plant with mesophytic foliage (e. g. the colocynth), a succulent 
tropophytic plant with delicate foliage (e. g. species of Senecio ), and numerous 
typical xerophytes. 
That the xerophytism of Coniferae is partly architectural in nature is 
demonstrated by several facts :— 
i. Though the individual leaf is small the aggregate leaf-surface of the 
conifer often greatly exceeds that of the dicotylous tree. 
In Table I, column 2, it will be seen that with specimens of equal age 
or size, the trees showing the largest aggregate leaf-surface are the Coniferae. 
(All the trees used by Von Hohnel in 1878 are described by him as being 
five to six years old and about 70 centimetres high.) The trees with the 
largest leaf-surface were Picea excelsa (14,250 sq. cm.), Abies pectinata 
(12,400), Pinus sylvestris (5,300) ; and the dicotylous trees most nearly 
approaching these were Acer platanoides (4,500) and Alims campestris 
