254 Groom.—Remarks on the Oecology of Coniferae. 
Ranging these statistics in order, by reckoning the largest as i and 
so forth to 8, which is the smallest, the position taken by the respective 
trees as regards transpiration and calibre of vessels is as follows :— 
Transpiration per grm. 
dry weight. 
Transpiration per square 
centimetre. 
Calibre 
of vessel. 
00 
00 
1879. 
Maximum. 
1878. 
j 879 - 
Maximum. 
i. Betula alba . . . 
I 
3 
5 
1 
1 
2 
4 
2. Fraxinus excelsior . 
, a 
1 
2 
2 
2 
3 
3 
3. Fagus sylvatica . . 
4 
2 
1 
[ 4 
3 
1 
7 
4. Acer Pseudoplatanus 
6 
6 
3 
l 3 
4 
4 
5 
5. Carpinus Betulus 
3 
4 
4 
[5 , . 
7 
6 
8 
6. Quercus Cerris . . 
8 
7 
6 
r® 7) 
6 
5 
1 
7. Acer plaianoides . . 
5 
8 
7 
16 ( 7 ) 
8 
7 
6 
8. Quercus Robur . . 
7 
5 
8 
8 
5 
8 
2 
Where square brackets point to pairs of numbers these approach 
equality. According to calculations both by surface and by weight the 
three trees transpiring most rapidly are Betula , Fraxinus , and Fagus. Of 
these, only one, Fraxinus , has vessels which come within the first three for 
size. While the lowest four as regards transpiration include Quercus Cerris 
and Q. Robur , which have the largest wood vessels. When the larch is 
included in the series, the lack of proportion between diameter of the 
conducting tubes and rate of transpiration becomes still more obvious. 
(c) Why have the Coniferae preserved the Tracheidal 
Structure of their Wood? 
As tracheae 1 have been evolved among Pteridophyta and occur in 
gymnospermous forms, we are not justified in assuming that the primitive 
Coniferae found it impossible to give rise to tracheae. The mechanism is 
such that it suffices not only for slowly transpiring leaves of evergreen 
conifers, but also for the rapidly transpiring ones of the larch. 
There are, in fact, some reasons for believing that the tracheidal 
mechanism is better suited than would be a tracheal mechanism with wide 
vessels to normal coniferous habits (including the structure and slow 
working of the evergreen leaf and the root). Though it has been indicated 
above that we are not justified in believing that there is a universal 
proportion between rate of transpiration and calibre of tracheae, there is no 
doubt that in some cases the method of widening or narrowing the tracheae 
is adopted when the transpiration increased or decreased respectively. This 
may show itself in the different shoots of one individual, where one is 
a branch-spine with reduced foliage and the other is a foliaged branch ; for 
instance, in the spines of Randia dumetorum (Groom, ’ 92 ) the vessels are 
narrower and scantier. Lothelier (’ 93 ) found the vessels and parenchyma 
less developed in spines than in foliaged branches. 
1 In this paper the terms ‘ trachea ’ and ‘ tracheal ’ solely refer to wood vessels. 
