56 THE MEDALS OF CREATION. Cuar. IV. | 
are rounded or conical, and their sides marked with trans- 
verse lines, rings, or bars. Their functions appear to be 
different from those of the spiral vessels, and they are found 
in situations where the latter never occur. . 
The organization of the stem in the whole class of flower- 
ing plants, possesses characters so evident, as to afford the 
most important aid in the investigation of their fossil 
remains. Without dwelling on minor modifications, they 
are separable into two divisions, namely, the Endogenous 
(signifying to grow from within), and the Exogenous (to 
grow from without). Both possess vascular tissue, but so 
differently arranged in the two classes, as to constitute dis- 
tinctive characters which are seldom obliterated, although 
what was once a flexible stem, is now a mass of flint. 
Enpoaenous Stems. —As the seeds of the plants belonging 
to this division have but one cotyledon, or seed-lobe, as the 
Lily, they are also termed monocotyledonous ; the reader 
therefore must remember that these terms are synonymous. 
These stems consist of an uniform mass of cellular tissue, 
in which bundles of vascular or woody fibre are imbedded ; 
a transverse section presents a surface dotted over with spots, 
produced by the division of these groups of vessels, pretty 
uniformly distributed, but more densely arranged towards 
the circumference (Lign. 1, fig. 4). A slice of cane affords 
an illustration of this structure. 
The increase of these stems is effected by the formation 
of new cells and bundles of vessels in the central axis, which 
force their way among the old tissue, and occasion the con- 
densation of the latter towards the outer edge. These plants 
have neither pith, concentric circles of woody fibre, nor true 
bark ; negative characters of the highest importance in the 
determination of fossil stems. 
ExocENous Stems.—The seeds have two cotyledons, or 
seed-lobes, as in the Bean, hence the plants of this class are 
