The Use of Anatomical Characters. 
portions of the petiole, and it is advisable to examine a basal, a 
median and an apical transverse section in every case. 
One of the most prominent features of the lamina and one 
which has long been employed by systematists, is the hairy 
covering. The constancy of a certain type of hair throughout 
entire orders ( e.g . stellate hairs of Tiliaceae, two-armed hairs of 
Sapotaceac) is probably generally due to an inherent property in 
the plant owing to its ancestor or ancestors having had this kind of 
hair; under similar external conditions it is clearly retained, as in 
the majority of holopetalous Tiliaceae, whilst in the Sapotaceae, for 
example, a considerable amount of variation on the two-armed type 
occurs without its ever becoming absolutely obscured, i.c. this type 
is recognisable as the basis from which all the other forms of hairs 
found in the order have been derived. On the whole, however, the 
hairy covering, except in cases like the one just mentioned, in which 
all the hairs, occurring within the limits of an order, are reducible 
to one general type, is not a character of ordinal value, although of 
the utmost importance in specific or even generic distinction. 
Glandular hairs, as being more intimately connected with the 
physiological processes in the plant, which are probably almost 
identical in related groups, may be expected to afford far more 
constant characters, as is in fact the case. In the first place the 
mere fact of their occurrence or absence is generally a feature 
characteristic of entire orders, and then these external glands 
commonly preserve a very uniform type of structure throughout 
large groups, thus serving as a general character, rather than as a 
generic or specific one. The kind of mineral secretion (oxalate of 
lime, carbonate of lime, silica), as well as the form in which it occurs, 
is again undoubtedly of very considerable importance as an 
indication of affinity and for the diagnosis of genera or even orders, 
whereas the distribution and abundance of the same can rarely be 
even employed for specific distinction ; in some cases the distribution 
has however been shown to be of some value, notably when one 
form of secretion occurs in the axis, whilst another appears in the 
leaf. The characteristic structure of the stomatal apparatus in 
many orders is undoubtedly due to an inherent property in the plant 
and is well-known to be of great importance in the characterisation 
of some orders (Rubiaceae, Cruciferae, etc.) 
There are few other characters of the leaf that are of any great 
importance, although a number may in some cases serve for specific 
distinction. Of these we may mention the occurrence and 
