340 PRINCIPLES OP NATURAL ARRANGEMENT. 



Cryptogamia (which have been hence termed Cellular es). 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 Coniferce and allied orders (. 78), together with the 

 absence of the dotted-ducts or special sap vessels, is charac- 

 teristic 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 subdivisions of the classes 

 of Plants. 



494. The structure and mode of increase of the Stem afford, 

 as already stated, the means of establishing the soundest division 

 of the Phanerogamia : and the two groups of Exogens and 

 Endogens are universally recognised as natural classes. Between 

 these, however, there are several connecting links, some Exo- 

 gens exhibiting in their stem no separation into annual layers, 

 and some Endogens, presenting an approach to the Exogenous 

 division of the kingdom. One small order (Calycanthece) is 

 known by the presence of four incomplete centres of vegetation 

 surrounding the principal one ; and the Passion-flower tribe are 

 remarkable for having the stem almost cut into four quarters : 

 whilst a square stem is universal in the Dead- Nettle tribe. In 

 some orders, such as the Cactece (Prickly-Pear tribe) and Euphor- 

 biacecB (Spurge Tribe), the quantity of cellular 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 classes 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. As a general rule, however, it may be 

 observed that neither bulb nor rhizoma (. 149, 150) are found 



