STRUCTURE.] THE STEM. 165 



very compound hardened hairs. They have no connexion 

 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 they are a development of 

 the epiphloeum of the bark. According to Dutrochet 

 (Memoir es, i. 174), it is exclusively by the base where the 

 epiphlceum and prickle are in contact, that the development 

 takes place of cells to increase the prickle in size. In the 

 Rose the prickle is formed in one year, and afterwards dies. 

 In Xanthoxylon 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, sometimes called 

 polarity, is found in all embryos, properly so called, there 

 being no known exception to it ; and the tendency is more- 

 over so powerful, that, as we shall hereafter see (Book II.), 

 the most "powerful external influence is rarely sufficient to 

 overcome it. The result of this development is the axis, or 

 centre, round which the leaves and other appendages are 

 arranged. That part which forces its way downwards con- 

 stantly avoiding light, and withdrawing from the influence of 

 dryness, 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 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 the cotyle- 

 dons belongs to the stem. As the double elongation just 



