26 INTRODUCTION, 
the primary divisions,—the presence of spiral vessels being on the whole 
characteristic of flowering Plants (which have been hence termed Vas- 
culares); and their absence being nearly constant in Cryptogamia (which 
have been hence termed Cellulares.) There are some of the inferior 
Phanerogamia, however, in which no spiral vessels can be detected ; and 
in the Ferns which stand at the head of the Cryptogamia, modifications 
of them may be found. However, if on examining any portion of 
the fabric of an unknown plant, spiral vessels were distinctly seen this 
might be regarded as sufficiently indicating that the specimen belonged 
to the higher of these two groups. The peculiarity of the woody fibre 
in the Conifere and allied: orders, together with the absence of the dot- 
ted-ducts or special Sap vessels, is characteristic of that portion of the 
Phanerogamic division; but excepting in this instance, no use can be 
made of the varieties of the elementary tissues, in defining the subdivis- 
ions of the classes of Plants. 
29. The structure and mode of increase of the stem afford as already 
stated, the means of establishing the soundest division of the Phaneroga- 
mia: and the two groups of Exogens and Endogens are universally recog- 
nized as natural classes. Between these, however, there are several 
connecting links—some Exogens exhibiting in their stem no separation 
into annual layers,—and some Eudogens, presenting an approach to the 
Exogenous division of the kingdom. One small order ( Calycanthee) is 
known by the presence of four incomplete centres of vegetation surround- 
ing the stem almost cut into four quarters; whilst a square stem is uni- 
versal in the Dead-nettle tribe. In some orders, such as the Cactee 
(Prickly-pear-tribe) snd Euphorbiacee (Spurge tribe), the quantity of cel- 
lular tissue usually so much predominates that the stems are soft and 
succulent ; but this is not always the case, some genera having stems of 
the ordinary character. No very positive characters can in general 
therefore, be drawn from the structure of the stem, in dividing the clas- 
ses into sub-classes and orders. Nor do the roots afford any better guide; 
since the modifications of form of which they are susceptible are very 
few, and they are by no means constant in particular groups. Asa 
general rule, however, it may be observed that neither bulb nor rhizoma 
are found in Exogens, and that they are confined to a few orders among 
Eudogens. 
30. The leaves are subject to considerable modifications, both in posi- 
tion, form and structure, which are very useful in classification. The 
general differences among the leaves of Exogens, Endogens and Acro- 
gens have already been adverted to. The relative position of the leaves, 
as whether alternate, opposite, or verticillate, is often a very important 
character, but in regard to this, as well as to other characters, it often 
