known by the number of concentric circles of the wood. But this rule is of uncertain application, owing to numerous disturbing causes, especially in countries in which the period of rest is less distinctly marked than in the winter of northern latitudes.

116. The secretions of plants are deposited most abundantly in the oldest concentric layers; while those layers which are most recently formed contain but a slight deposit.

117. When the tissue of the concentric layers is filled with secretions, it ceases to perform any vital functions.

118. The dead and fully formed central layers are called the heart-wood.

119. The living and incompletely formed external layers are called the alburnum.

120. Upon the outside of the wood lies the BARK, which, like the wood, consists of concentric layers.

121. It consists of four distinct parts: 1. the Epidermis 66 a; 2. the Epiphlæum 65 66 b ; 3. the Mesophlæum 65 66 c the Endophloum or Liber 65 66 d

65 66 c; and 4,

122. Each of these parts increases by successive additions to its own inside, except the epidermis, which is never renewed.

123. The Epiphlæum and Mesophlæum are both formed of cellular tissue only; but their cells are placed in different directions with respect to each other. The former is often large and soft, and may separate spontaneously from the young

Mistakes

layers forming beneath it, as in Cork, which is the epiphloum of Quercus Suber.

124. The Endophlæum or Liber consists of cellular tissue resting on the alburnum, of laticiferous tissue (36), and of pleurenchyma (23). The tubes of the latter are often thickened rapidly by a deposit of sedimentary matter; in which case, sections of the tubes present the appearance of concentric circles 64 65 66 Hence arises the toughness of the tubes of pleurenchyma which occur in the liber, and are manufactured into cordage, as in the Lace-bark tree, the Lime-tree, &c. 125. Occasionally the liber is only formed during the first

Sundays year's growth; after which it is enclosed in wood, and is

eventually found near the pith. This has as yet been observed only in the Menispermaceous order.

126. The power of renewing themselves by the production of new matter upon their inner surface, is apparently given to the layers of bark in order to compensate for the gradual and incessant distension of the wood beneath them.

127. As the older parts die, from becoming too small to bear the strain upon them, new parts form, each in its allotted place, and take the station of that which went before it.

128. The secretions of a plant are often deposited in the bark in preference to any other part.

129. Hence chemical or medicinal principles are often to be sought in the bark rather than in the wood.

130. The immediate functions of the bark are to protect the young wood from injury, and to serve as a filter through which the descending elaborated juices of a plant may pass horizontally into the stem, or downwards into the root.

131. It also contains the laticiferous vessels (36), by which the latex is conveyed to all parts of the surface of a plant.

132. The MEDULLARY RAYS or PLATES consist of compressed parallelograms of cellular tissue (muriform cellular tissue), belonging to the medullary system.

133. They connect together the tissue of the trunk, maintaining a communication between the centre and the circumference.

134. They act as braces to the woody and vasiform tissue of the wood. They convey secreted matter horizontally from the bark to the heart-wood, and they generate adventitious leaf-buds.

135. Cambium is a viscid secretion, which, in the spring, separates the alburnum of an Exogenous plant from the liber. It is free vegetable mucilage, out of which the new elementary organs (8) are constructed, whether in the form of vessels, or woody tissue, or of the cellular tissue of the medullary system, whose office is to extend the medullary plates, and maintain the communication between the bark and central part of a stem.

136. As Exogenous plants increase by annual addition of new matter to their outside, and as their protecting integument or bark is capable of distension in any degree, commensurate with the increase of the wood that forms below it, it follows, taking all circumstances into consideration, that there are no assignable limits to the life of an Exogenous tree.

137. The stem of ENDOGENOUS plants offers no absolute distinction of Pith, Medullary Rays, Wood, and Bark.

138. It is formed by the intermixture of bundles of vascular tissue among a mass of cellular tissue, the whole of which is surrounded by a zone of cellular and woody tissue, inseparable from the stem itself, and therefore not bark.

139. It increases by the successive descent of new bundles of fibro-vascular tissue down into the central cellular tissue, curving outwards as they descend.

140. The vascular bundles of the centre gradually force outwards those which were first formed, the cellular mass augments simultaneously, and in this way the diameter of a stem increases.

141. What appears to be bark in these plants is an external layer of cellular tissue, into which the lower extremities of the arcs of fibro-vascular tissue descend obliquely, losing their vascularity as soon as they reach the cortical integument, or falsebark.

142. It is in consequence of this continuity in an oblique direction of the fibro-vascular bundles and the external cortical integument, that the latter can never, in Endogens, be separated from the wood beneath it.

143. The diameter of the stem of an Endogenous plant is determined by the power its tissue possesses of distending, and by its hardness.

144. When the external tissue has once become indurated, the stem can increase no further in diameter.

145. When the tissue is soft and capable of continual dis

C

tension, there is no more certain limit to the life of an Endogenous than of an Exogenous tree.

146. Generally, the terminal bud only of Endogenous plants is developed; but very often a considerable number develope; Ex. Asparagus.

147. When a terminal bud only of an Endogenous plant developes, the stem is cylindrical; Ex. Palms: when several develope, it becomes conical; Ex. Bamboo.

148. In Acrogens no other stem is formed than what arises from the simple union between the bases of the leaves and the original axis of the bud from which they spring, and which they carry up along with them.

149. In the order of Ferns the section of a stem indicates the same structure as that of the numerous petioles (197) out of which it is constituted.

150. When Acrogens have no proper leaves, they are mere expansions of cellular matter, sometimes in all directions; Ex. Fungi: sometimes in particular directions; Ex. Lichens, Algæ, &c.

151. The stem of a plant assumes numerous and very different appearances in different plants.

If above ground it is rootshaped, or knotted 67; ascending 68; creeping72; articulated 73; leafless, succulent, and deformed 69 ; or leafy 71.

If it bears the flowers, proceeding immediately from the soil or near it, it is a scape 70.

152. It often burrows beneath the earth, when it is vulgarly called a creeping roots. Sometimes the internodes become much thickened, when what are called tubers77 78 are formed; or the stem lies prostrate upon the earth, emitting roots from its under side, when it is called a rhizoma, or rootstock 67.

153. If it distend underground, without creeping or rooting, but always retaining a round or oval figure, it is called a corm 74 75

154. All these forms of stem are vulgarly called roots.

155. No root can have either scales, which are the rudiments of leaves, or nodes, which are the rudiments of buds. A scaly root is, therefore, a contradiction in terms.

156. The ascending axis, or stem, has nodes and internodes. 157. Nodes are the places where the leaves are expanded and the buds formed.

158. Internodes are the spaces between the nodes.

159. Whatever is produced by the evolution of a leaf-bud (164) is a branch.

160. A spine is the imperfect evolution of a leaf-bud, and is, therefore, a branch.

161. All processes of the stem which are not the evolutions of leaf-buds, are mere dilatations of the cellular integument of the bark. Such are prickles (61).

162. In solid form the stem is extremely variable; the following are common terms relating to it :