116 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 
If we examine the structure of plants, we find that it is com- 
posed of minute cells, formed by an elastic transparent substance 
called cellulose, having a chemical formula of C24,H20,010. 
This is considered the base of vegetable tissue, of which there 
are two kinds, cellular and vascular. These constitute the 
elementary organs, and by their union in various forms the 
different functions of plant-life are carried out. 
Botanists divide the vegetable kingdom into four great 
divisions, viz.,.—Exogens, Endogens, Acrogens, and Thallogens. 
The exogenous plants include all trees and shrubs from the 
tiniest heath to the great Wellingtonia gigantea. The stem of 
Exogens, as a rule, consists of six parts, viz, (1) epidermis, (2) 
epiphleum, (3) mesophleum, (4) liber, (5) alburnum, and (6) 
duramen or heart-wood. Plate 1, which shows transverse 
section of the stem of the Upas Tree of Java, Antiaris toxicaria, 
is a very good example of this. The four outer parts are, 
generally speaking, composed of fibro-cellular tissue, and are 
blended into one mass, with the liber resting on the alburnum. 
All Exogens have a pith and medullary rays. Endogens, on the 
other hand, show no pith, medullary rays, wood, or bark in their 
stems, but are formed of an intermixture of bundles of fibro- 
vascular tissue, all held together by a zone of woody tissue 
attached to, and inseparable from, the stem. A typical 
illustration of this is the section, shown on Plate 2, of the 
stem of the Maize, which is almost identical with the Bamboo. 
These plants increase their bulk by the distention of the 
vascular bundles of the centre forcing outwards the bundles which 
were first formed. These latter, by and by, lose their vascularity 
and become the cortical integument or woody zone in all this 
division. Plate 3, the leaf stalk of Pteris serrulata, represents 
the structure of the Acrogens, which includes all the ferns, It 
will be observed that their structure is very distinct from that 
of both the Exogens and the Endogens. Plate 4 shows an 
example of the Thallogens, which form the lowest order of 
vegetable life. This plate shows Podisoma juniperina in the act 
of growing from a fracture in the bark of the Juniper. As a 
rule these plants are parasitic, and are generally mere expansions 
of cellular tissue, and they may be considered one of the great 
scavengers provided by Nature for reducing matter to its natural 
elements. 
In dealing further with the subject, we will confine ourselves 
