THE NATURAL SYSTKII OF CLASSIFICATION. 371 



frond, composed of parenchyma alone : fertilization giving rise to a 

 sporangium filled with spores. 



727. The remaining and lower grade consists of plants such as 

 Lichens, Seaweeds or AlgJB, and Fungi, which exhibit no clear dis- 

 tinction into stem, root, and leaves, but consist of single cells or rows 

 of cells in their lowest grades, and in the higher, of masses of cells 

 disposed in almost every shape, but tending mostly to flat strata or 

 expansions ; hence the vegetation is ^termed a thallus (or bed), and 

 this word gives a name to the class, viz. 



Class V. Thallophytes : cellular Cryptogamous plants with 

 no distinction of axis and foliage ; their spores mostly directly fer- 

 tihzed (as explained in another place, 656-661). 



728. These five classes are unequal in extent and diversity ; the 

 Exogenous class containing much the largest number of orders ; the 

 Endogens also comprising a considerable number ; the others com- 

 prise few orders or main types, but are most of them very rich 

 in tribes, genera, and species. 



729. Only the first or highest class presents such marked diver- 

 sity of type among the plants it comprises as to caU for the estab- 

 hshment of subclasses, that is, of groups of such importance as to 

 raise the question whether they should not be regarded as classes. 

 This question is raised by the peculiarities of Coniferoe (Pines, Cy- 

 presses, the Yew, &c.), and by the tropical order of Cycadacea; ; in 

 which, not only are the flowers reduced to the greatest simplicity, 

 but the fertile ones consist of naked ovules merely, borne on tlie 

 margins or surface of a sort of open leaf, or else of an ovule, without 

 anything answering to a pistil at all. But as these plants are truly 

 exogenous and dicotyledonous (or often polycotyledonous), the better 

 opinion is that they should be ranked under the Exogenous or 

 Dicotyledonous class, as a subclass. So that, while the main body 

 of the first class consists of 



Subclass I. Angiosperjious Exogens : viz. those with 

 proper pistils enclosing their ovules in an ovary, in the ordinary 

 manner ; the pollen to fertilize the ovules received upon a stigma 

 (420, 559, 574), — the others form the 



Subclass II. Gymnosperjious Exogens: those with naked 

 ovules and seeds (as the name denotes), M'hich are fertilized by 

 direct application of the pollen (560, 573, 625). 



730. The general plan of the classes and subclasses may be pre- 

 sented in one view, as in the subjoined synopsis. 



