GARDENING AS A SCIENCE. 
59 
of the gfeat distinction 
Fig I. 
The annexed figure may serve to convey some idea 
which exists between stems of the two classes already 
described. Fig. 1. is part of the upright stem of an 
exogenous plant : is the bark covered by its epidermis 
or cuticle ; b point to the concentric circles of fibrous 
vessels that surround the central medullary sheath, and 
pass downwards and upwards amidst the radiating con- 
vergent processes. 
Fig. 2. represents a very thin horizontal slice of the 
growing sugar cane, a noble plant of the second, or 
endogenous class. Here ccc^ show the bundles of ligneous 
fibres which pass down the stem from its apex to the 
collar, interspersed among a mass, d d d, oi cellular tissue, 
consisting of irregular six-angled cells so piled one above 
the other at the angles, as to resemble, in a degree, the 
cells of the honey-comb. We need hardly say, that these 
cells are the depositaries of the saccharine juices of the 
cane. 
The wood of endogens is thus seen to be different in 
structure and position from that of exogens. It may be 
questioned, indeed, whether the terms wood and bark can be admissible ; although 
no one will attempt to assert that the chemical constituents of true wood, and 
of the Jihro-ligneous appendages, in endogens, is not identical. However, as 
in the higher tribes of this class — the Cocos, the Palms, the Pandanus, &c., &c. 
— stems are ultimately formed ; we may admit, with some qualification, the 
existence of wood, in the abstract ; and it now remains to show the manner in 
which this stem is progressively formed. 
As the cellular tissue extends in every direction, the bundles of woody fibres 
curve outwards as they descend, their lower extremities terminating in a multitude 
of threads at the circumference, where they become a cortical integument" repre- 
senting bark, but being, in reality, a closely-packed, hardened series of fibrous 
threads, of which the interior coating of a cocoa-nut is not an inapt type. This 
external integument differs more or less in trees of different genera ; but, in all, 
while the external circumference hardens, the interior of the stem remains compara- 
tively soft and open, being composed of a large proportion of cellular substance. 
On this subject, the authority already cited observes, when speaking of old palms, 
" that the woody bundles next the circumference are larger and harder than they 
originally were, and, consequently, we must suppose that they have the power of 
increasing their own diameter subsequent to their first formations : and that they 
also act as reservoirs of secretions of a hard and solid nature, after the manner of 
the heart-wood of exogens. 
" When the growth of the stem of an endogen goes on in this regular manner, 
