182 
The Use of A natomical Characters. 
of secretion—are often constant throughout an entire order, 
whilst their distribution throughout the various tissues of the stem, 
as well as their shape, afford valuable characters for smaller groups 
within the limits of these orders; as examples it is sufficient to 
mention the mucilage-receptacles of the Tiliaceae and the resin- 
canals of the Anacardiaceae. The universal occurrence of latici- 
ferous tissue in some orders has long been known and employed in 
characterising them, whilst Papaveraceae and Celastraceae show 
that even this character may be present or absent in closely 
related genera or even species. In spite of such exceptions (which 
may be due to a polyphyletic origin of the order), the occurrence of 
definite secretory organs must in my opinion be regarded as a 
character acquired long ago, although their distribution, shape, etc. 
is naturally variable with the conditions of habitat. The fact that 
the laticiferous elements for example are, in some orders, already 
differentiated at a very early stage in the embryo, indicates their 
phylogenetic importance and that they are not merely a recently 
acquired adaptive feature; far too little is known of the stages at 
which other types of secretory organ first appear, but a comparative 
study of this point would be most interesting. 
The different features of the axis may be summarised, from the 
point of view of their systematic importance, in the following way:— 
Characters usually generic or 
ordinal. 
vS tincture of pitli. 
Perforations of vessels. 
Pitting of vessels and wood- 
fibres. 
Breadth of medulfaiy rays. 
Cork-development. 
Type of secretory organs. 
Characters mostly only specific. 
Occurrence of isolated groups 
of sclereiichyma in pith and 
cortex. 
Number and arrangement of 
vessels of wood. f 
Distribution and amount of 
wood-parenchyma. 
Distribution of secretory 
organs. 
Occurrence and arrangement of 
bast-fibres. 
With respect to the leaf the anatomical features of the petiole are 
largely dependent on those of the lamina, especially the distribution 
of the vascular tissue in the former, which varies considerably with 
the size of the leaf. As far as our present knowledge goes, petiolar 
characters are thus mostly only of specific value (but it seems likely 
that they may turn out very useful in this respect); however in the 
Dipterocarpeae they are constant throughout and the same is the 
case in the genera of Salicineae and Cupuliferae. The arrangement 
of the vascular system often presents marked differences in different 
