The Use of Anatomical Characters. 181 
number of important points: the perforation of the vessels 
whether simple or scalariform, must be named first, the former 
character especially being constant for many orders, whilst scalari¬ 
form perforations are characteristic of many genera, rarely of an 
entire order. The mode of perforation is evidently an inherent 
property of the plant—at least it is difficult to conceive of its 
dependence on external conditions—and must therefore necessarily 
he a feature of great systematic importance. The same is true of 
other characters of the wood, i.e ., the pitting of the wall of the vessels 
and of the fibres of the wood, and also the breadth of the medullary 
rays, hut other features again—number and arrangement of the 
vessels, distribution and amount of the wood-parenchyma, shape of 
the cells of the medullary rays—will vary with the conditions of 
habitat, are only of specific value, and not even that if the 
species is capable of living in varying localities. The cortex, except 
for the one excellent feature, afforded by the point of origin of the 
cork, is subject to more variation within closely related groups than 
the other portions of the axis, The degree of development and the 
distribution of the fibres of the bast, of which much has been made 
by some investigators, is a character varying very much with the 
habitat; the fact that it affords a more or less constant feature in 
the Tiliaceae and some other orders, is probably due to similarity 
of conditions of life in the members of the order. These two factors, 
both of which cause constancy in anatomical structure, must always 
be kept in mind, i.e., similarity of conditions of growth and the ten¬ 
dencies of the plant due to its ancestry; it will frequently be 
exceedingly difficult to separate the two, but where such a dis¬ 
tinction can be drawn, the latter character should always be 
accepted as more applicable in the systematic characterisation of 
larger groups of affinity. Another feature of the cortex, which is 
undoubtedly also of great systematic importance, but must still be 
considered as dependent on external conditions, is the occurrence 
of a sclerenchymatous ring at the external limit of the primary 
bast (in the “pericyle ” of Van Tieghem). 
One of the most essential points of the structure of the axis 
in many orders still remains to be discussed and that is the occur¬ 
rence of secretory organs. Although almost invariably also repre¬ 
sented in the stem when they occur in the leaf, the reverse is not 
always true and thus an examination of the stem-anatomy is 
necessary for the determination of this all-important feature. 
Certain types of secretory organs—commonly with a definite kind 
