40 THE VEGETABLE WORLD. 
letters T, FL, v, and the pith by the letter Mm. To make this more 
clear to the reader, we avail ourselves of an illustration borrowed 
from Mr. Christopher Dresser’s “ Rudiments of Botany ” (Fig. 48), 
in which a is a horizontal section of an exogenous stem, that is, 
of a stem consisting of pith, 
Ls wood, and bark, which is 
enlarged by external addi- 
tions. abc are the cellular 
mass of the stem, in which 
bundles of woody fibre, ¢, 
(| have been deposited. By 
yi) W\\-~" the disposition of these bun- 
WIP NWi\\\ dies, the cellular matter has 
/ whe i Hr become divided inte a 
HIN central portion, a, which 1s 
| AM iy ) 
~ i NW} pa \ if 
the pith, an outer portion, ¢, 
which is the bark, or corticle, 
: . and the plates, 4, intervening 
between the bundles of wood- 
tions of an exogenous st fibre which connect the bark — 
and the pith; these are the 
medullary rays or plates. 8 is the section of an older stem, in 
which a bundle of woody fibre has been deposited between each of 
the older bundles. a, pith, 6, medullary rays, ¢, cortex, or bark, 
4, bundles of wood-fibre. | 
The vertical section c represents a portion of a stem with the 
bark removed, showing that the bundles of fibre d do not descend 
vertically through the cellular matter 4, but in waved lines, 
dividing the cellular into lenticular masses, which form the medul- 
lary rays or plates 4. : 
Most trees are provided with medullary rays of only one kind. 
A few only present thick and thin rays together. Thus in the 
Oak or the Hornbeam we find both thick and thin rays, whilst in 
Willow and Maple the rays are visibly equal. 
Whilst the fibres and vessels never contain solid nutritive 
matter, the cells, which, by their junction, form the medullary . 
Age the seat of an abundant production of small granules of 
