178 
RUDIMENTS OF THE NATURAL SYSTEM OF BOTANY. 
culares, called by some Pkceno^amous, or flowering plants, are those which, while 
they possess a cellular tissue in common with Cellulares, have also a system of 
tubular, longitudinal, and straight or spiral vessels. Cellulares^ on the other 
hand, are composed of cells alone, the form of which is always some modification 
of round, the angular corners being caused by compression. These are the most 
important distinctions ; and while it is more easy for the learner to consider the 
former as flowering^ and the latter as Jlowerless plants, it is highly useful to fix in 
the memory the scientific appellations of each, because these afibrd an excellent key 
to their structure. 
In the class Vasculares^ nearly all the plants which possess any interest to the 
cultivator are arranged ; Cellulares being, for the most part, inconspicuous and 
worthless. Vascular es will, consequently, claim primary and most extensive con- 
sideration. They are first divided into two sub-classes, respectively denominated 
ExoGENJE and ENDOGENiE, to which the terms Dicotyledoneje and Monocotyle- 
DONEJE, are concurrent. The titles first mentioned refer to the mode of growth ; 
the stems of ExogencE enlarging exteriorly, and those of EndogencB towards the 
centre. The two other terms contain an allusion to the number of seed-lobes, 
moms and dis signifying one and two. 
Exogens principally inhabit temperate climes, and are much more numerous 
than Endogens. In the absence of seeds, they may be known by their stems 
being branched, and having both pith and bark. With herbaceous plants, or even 
trees, a more simple method of determining their class is an examination of the 
leaves ; those of Exogens being reticulated, that is, having a great number of irre- 
gular transverse or divergent veins. It should be remembered, however, that 
many Endogens have parallel veins to their leaves, and that it is only when these 
branch laterally, and ramify themselves throughout the entire leaf, that they are 
to be regarded as characteristic of Exogens. 
The common trees of our own climate may be mentioned as examples of this 
sub-class. In the centre of all these, especially while young, a greater or 
less circular deposite of pith may be found. This is composed solely of cellular 
tissue, and may also be distinguished by its remarkable elasticity, by its suscepti- 
bility of compression, and, compared with the surrounding wood, its extreme 
softness, while, after the wood is duly formed, the substance in question ceases to 
contain fluids, and is, in fact, a mere mass of cellular integument. That, in such 
a state, the pressure of exterior deposites of wood should contract its bulk, is not 
at all surprising ; particularly if, as physiologists assert, a horizontal communica- 
tion, for the conveyance towards it of annual secretions, is constantly maintained 
in Exogenous plants. Accordingly if a tree is examined at different epoclis of its 
age, the observer will discover that the volume of pith is gradually lessened, year 
after year, till, in old trees, it is scarcely perceptible. 
Let it not be supposed that this information is misplaced or superfluous. One 
of the acknowledged tests of Exogenous plants is the existence of pith ; and, 
