140 
GROWTH OF PLANTS. 
medullary rays ; and their pith, instead of being confined to the 
centre of the stem, extends almost to the circumference. The 
wood is divided into fibres, running longitudinally through the 
stem ; each of these fibres seems to vegetate separately ; they 
are ranged around a central support ; and are so disposed that 
the oldest are crowded outwardly by the developement of new 
fibres in the centre of the stem ; this pressure causes the exter- 
nal layers to be very close and compact. This mode of increase 
little favorable to growth in diameter, produces long and straight 
stems which are nearly uniform in their size, throughout their 
whole extent ; as the palms and sugar-canes of the tropics, and 
the Indian corn of our climate. Most of these plants present 
us with roots of the fibrous kind. 
Dicotyledons are called exogenous plants, because their stems 
grow from without; that is, the wood grows externally, and 
each succeeding layer presses upon the central ones ; thus the 
exogenous plants are more hard and compact towards the centre ; 
as in the oak ; while the endogenous plants are more firm in 
their external parts ; they continue to increase in height, long 
after they cease to grow in diameter. The stem is gradually 
extended upwards by new terminal shoots, which are formed 
annually. The epidermis is formed of the foot stalks of leaves 
which annually sprout from the rim of a new layer of wood ; 
the leaves falling in autumn, their foot-stalks become indurated 
and incorporate themselves into the outer surface of the plant. 
We have now taken a brief view <5f the most important facts 
and principles which constitute the science of vegetable physiol- 
ogy including anatomy. That you may have found the subject 
somewhat tedious, need give you no apprehension that your 
minds are not fitted for minute investigations. The most scien- 
tific observer of nature, cannot but feel, with yourselves, that after 
all his scrutiny, organic life is shrouded in mystery. Although 
the vegetable structure is less complicated than the animal, there 
are many analogies between them ; and many parts of the for- 
mer have been named, and various phenomena explained, by a 
reference to names and principles common to animal anatomy 
and physiology. You cannot therefore expect, at the first 
glance, to comprehend explanations which presuppose some 
knowledge of those intricate subjects. By attention to the veg- 
etable structure, you will, doubtless, be induced to think more 
upon the wonderful mechanism of your own material frames, 
and upon the great resemblance, yet infinite difference between 
yourselves and the trees of the forest, and the lilies of the field. 
You will, in the language of the Psalmist, be led to exclaim, 
Why Dicotyledons are termed exogenous — Formation of epidermis — 
