78 EXOGENOUS OR DICOTYLEDONOUS STEM. 
gradual, the inner part of each being first formed, and growth 
proceeding progressively to the outside: hence these also are 
progressive bundles; but, as such bundles have no special layer 
of generating cells resembling the cambium layer, no additions 
to them can be made in successive seasons, as is the case in the in- 
definite fibro-vascular bundles of Exogenous stems. Hence the 
new bundles are not developed in continuity with the old, but 
remain distinct and of limited size, and are therefore named 
definite or closed fibro-vascular bundles, 
In Acrogenous stems the jfibro-vascular bundles are chiefly 
made up of vessels of the scalariform, annular, or spiral type, 
according to the different orders of Cormophytes from whence 
they have been derived; these are surrounded by delicate 
tubular cells, and the whole is enclosed by a firm layer of 
parenchymatous cells the walls of which have undergone a 
thickening and hardening process, and to which the name of 
sclerenchyma has been given, and forming what has been called 
the bundle-sheath. Such bundles only grow by additions to 
their summit ; and as these bundles, like those of Endogenous 
stems, have no special layer of cambium cells, they are also said 
to be closed or definite. 
The distinctive appearances and modes of growth which we 
have thus seen to occur in the stems of the two Flowering 
Plants above noticed are also accompanied by certain differences 
in the structure of their embryo. Thus plants with Exogenous 
stems have an embryo with two cotyledons (figs. 16, ¢, c, and 
18, c, c); those with Endogenous stems have but one cotyledon 
in their embryo (fig. 19, c). Hence Exogenous stems are also 
termed Dicotyledonous ; and Endogenous stems Monocotyledon- 
ous. For reasons which we shall describe hereafter, the latter 
terms are in some cases to be preferred to the former. In the 
succeeding pages we shall use them indiscriminately. Acro- 
genous stems are also sometimes termed Cryptogamous, because 
they are only found in Flowerless plants. With these general 
remarks on the internal structure of the three kinds of stems 
we now proceed to describe them respectively in detail. 
A. ExoGENnous or DicoryLeponous StTEM.—AIll the trees 
and large shrubs of this country, and with rare exceptions those 
of temperate and cold climates, are exogenous in their growth. 
In warm and tropical climates such plants occur associated with 
those possessing endogenous and acrogenous structure; but 
Dicotyledonous plants are far the most abundant even in these 
parts of the world. 
In the embryo state, the Exogenous stem is entirely com- 
posed of parenchyma, But as soon as growth commences, some 
of its parenchymatous cells become developed into vessels and 
wood-cells, so as to form the indefinite fibro-vascular bundles 
which are characteristic of such a stem. ‘These woody portions 
(fig. 182, t) are at first separated from each other by large 
