130 STRUCTURE OF EXOGENS. 
are called medullary rays, and formed of cellular tissue like 
the pith. This is surrounded by a longitudinal canal, which is 
called the medullary sheath, and contains spiral vessels. 
Outside of this are the rings of wood, formed of vessels and of 
woody tissue; those near the centre, called the heartwood, 
are denser and more highly coloured than those which are 
more external. Of these, the youngest are known by the 
name of sap-wood, and are those most recently formed, as all 
additions of wood are on the outside of the growth of previous 
years, in these trees. 
On the outside of the layers of wood, we find the part called 
bark, but composed also of a series of layers; of these the 
oldest are on the outside, and may in some trees be seen 
splitting and scaling off in a withered and dead state. The new 
layer of bark will, however, be found in the inside of all, and 
next to the young wood. The structure of this part can be 
best examined in the bark of a young shoot. It will be found 
to be composed of two layers of cells, which receive different 
names from botanists, having the delicate epidermis or cuticle 
on the outside; but in the inside of all, the layer consists of 
tough, elongated cells and vessels, and is called liber or bast. 
This part is very conspicuous in the Lime tree of Europe, which, 
by the stripping off of its bark, yields the bast of which Russia 
mats are made. In the East, species of Grewia, of Hibiscus, 
and of Mulberry, are equally remarkable for their bast. 
Besides the above characteristics in the leaves, wood, and 
bark of Exogens, we may also observe distinctions in their 
seeds and flowers, by which they may also be readily dis- 
tinguished. Thus, the seeds of Exogens contain two seed- 
lobes, which, in sprouting, become the two seed-leaves, as seen 
in the germinating of Peas, Beans, Cucumbers, &c.; while in 
Endogens only a single seed-leaf is seen, as in the sprouting 
of Wheat and Barley, Rice, or the Date tree. In the flowers, 
also, distinctive characters may be observed; but we shall 
direct attention only to those of the Exogens, in which we may 
see the flowers composed of stamens and pistils, surrounded 
on the outside by floral envelopes. 
In some, these floral envelopes consist of two distinct kinds 
—the outer usually green and the inner variously coloured ; both 
consisting sometimes of several pieces more or less united, as 
