CHAP. II. 
STEM. 
69 
acute point. They may be, not improperly, considered as 
very compound hardened hairs. They have no connection 
with the woody tissue, by which character they are obviously 
distinguished from spines, of which mention will be made 
under the head of branches ; but are a developement of the 
epiphloeum of the bark. According to Dutrochet (Memoires, 
i. 174.), it is exclusively by the base where the epiphloeum 
and prickle are in contact, that the developement takes place 
of cells to increase the prickle in size. In the Rose the 
prickle is formed in one year, and afterwards dies. In Xan- 
thoxylon juglandifolium it is the produce of two or three 
years’ growth, according to the last mentioned author. Prickles 
are found upon all parts of a plant, except the stipules and 
stamens. They are very rarely found upon the corolla, as in 
Solanum Hystrix; their most usual place is upon the stem, 
as in Rosa, Rubus, &c. 
Sect. II. Of the Stem, or Ascending Axis, 
When a plant first begins to grow from the seed, it is a 
little body called an embryo, with two opposite extremities, 
of which the one lengthens in the direction of the earth’s 
centre, and the other, taking a direction exactly the contrary, 
extends upwards into the air. This disposition to develope in 
two diametrically opposite directions is found in all seeds, 
properly so called, there being no known exception to it ; and 
the tendency is moreover so powerful, that, as we shall here- 
after see (Book II.), the most powerful external influence is 
rarely sufficient to overcome it. The result of this deve- 
lopement is the axis, or centre, round which the leaves and 
other appendages are arranged. That part which forces its 
way downwards constantly avoiding light, and withdrawing 
from the influence of the air, is the descending axis, or the 
root ; and that which seeks the light, always striving to expose 
itself to the air, and expanding itself to the utmost extent of 
its nature to the solar rays, is the ascending axis, or the stem. 
The only exception to this is when the embryo first begins 
to grow. At that time the first part of the axis formed below 
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