IV. STEM.

67. The stem is produced by the successive developement of leaf-buds (142), which lengthen in opposite directions.

68. If an annular incision be made below a branch of an Exogenous plant (80), the upper lip of the wound heals rapidly, the lower lip not: the part above the incision increases sensibly in diameter, the part below does not.

69. If a ligature be made round the bark, below a branch, the part above the ligature swells, that below it does not swell.

70. Therefore the matter which causes the increase of Exogenous plants in diameter descends.

71. If a growing branch is cut through below a leaf-bud, that branch never increases in diameter between the section and the first bud below it.

72. The diameter of all Exogenous stems increases in proportion to the number of leaf-buds that is developed.

73. The greater the number of leaf-buds above a given part, the greater the diameter of that part; and vice versâ.

74. In the spring the newly forming wood is to be traced in the form of fibres descending from the leaf-buds; that which is most newly formed lying on the outside, and proceeding from the most newly developed buds.

75. Therefore the descending matter, by successive additions of which Exogenous plants increase in diameter, proceeds from the leaf-buds.

76. Their elongation upwards gives rise to new axes, with their appendages; their elongation downwards increases the diameter of that part of the axis which pre-existed, and produces roots.

76. a. Roots, therefore, in all cases, should consist of extensions of woody tissue; and this is conformable to observation.

77. Hence, while the stem is formed by the successive evolution of leaf-buds, the root, which is the effect of that evolution, has no leaf-bnds.

78. The leaf-buds thus successively developed are firmly held together by the medullary system of the stem, which proceeds from the bark inwards, connecting the circumference with the

centre.

79. The stem varies in structure in three principal ways.

80. In vascular plants it is either formed by successive additions to the outside of the wood, when it is called Exogenous; or by successive additions to its centre, when it is called Endogenous. In Cellular plants it is formed by the union of the bases of the leaves, and by addition to the point of the axis, or by simple elongation or dilatation where no leaves or buds exist, this is called Acrogenous.

81. The stem of EXOGENS may be distinguished into the Pith, the Medullary Sheath, the Wood, the Bark, and the Medullary Rays.

82. The PITH consists of cellular tissue, occupying the centre of the stem.

83. It never alters in diameter after it is once formed.

84. It is produced by the elongation of the axis upwards. 85. It serves to nourish the young buds until they have acquired the power of procuring nourishment for themselves.

86. The MEDULLARY SHEATH consists of spiral vessels.

87. It immediately surrounds the pith, projections of which pass through it into the medullary rays (113.).

88. It is in direct communication with the leaf-buds and the veins of the leaves.

89. It carries upwards the superfluous oxygenated air, either absorbed immediately from the earth, or obtained in part by the decomposition of carbonic acid (101.), and conducts it into the leaves.

90. The WOOD lies upon the medullary sheath, and consists of concentric layers.

91. It is formed by the successive adhesion of the descending axes of the buds, and by the interposition of the medullary system, here called medullary rays, connecting the pith and the bark.

92. The first concentric layer lies immediately upon the medullary sheath and pith, and consists of woody and vasiform tissue (18. a).

93. Each succeeding concentric layer consists of woody and vasiform tissue, which either form themselves into distinct strata, in which case the latter is innermost, or are confounded together.

94. When there is any material difference between the compactness of the tissue of the two sides of a concentric layer, zones are formed in which the woody tissue is outermost; but when the vasiform and woody tissues are equally intermingled, no apparent zones exist.

95. A concentric layer, once formed, never alters in dimensions. 96. Each concentric layer, which is distinctly limited, is usually the produce of one year's growth.

97. Therefore, the age of an Exogenous tree should be 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.

98. The secretions of plants are deposited first in the oldest concentric layers; while those layers which are most recently formed are either empty, or contain but a slight deposit.

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

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

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

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

103. Each concentric layer is composed of woody tissue, mixed with, and covered externally by, a layer of cellular tissue. 104. The woody tissue constitutes the liber.

105. The exterior cellular tissue constitutes the cellular integument or epidermis.

106. The concentric layers of the wood and bark are the reverse of each other, the former increasing externally, the latter internally. 107. The concentric layers of the bark are formed at the same period, and under the same circumstances, as those of the wood. 108. Therefore, the number of concentric layers in the one or the other is the same.

109. But while the concentric layers of the wood are imperishable except from disease, those of the bark are continually destroyed by the distension of the stem; and hence the bark is always perishing naturally, while the wood sustains no loss.

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

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

112. 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.

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

114. They connect together the tissue of the trunk, maintaining a communication between the centre and the circumference.1 115. 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 (153.). 116. Cambium is a viscid secretion which, in the spring, separates the alburnum of an Exogenous plant from the liber.

117. It is supposed to be destined to afford a proper pabulum for the descending fibres of the buds.

118. It is also, in all probability, the organising matter in which the cellular tissue of the medullary system is engendered, for the purpose of extending the medullary plates, and maintaining the communication between the bark and central part of a stem.

119. 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.

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

121. 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.

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

123. 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.

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

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

126. When the tissue is soft and capable of continual distension, there is no more certain limits to the life of an Endogenous than of an Exogenous tree.

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

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

129. In Acrogenous plants no other stem is formed than what arises from the simple union of the bases of the leaves to the original axis of the bud from which they spring, and which they carry up along with them. This subject is but ill understood.

129.a. When Acrogenous plants 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.

130 The ascending direction of the stem, upon its first developement, is frequently deviated from immediately after.

131. It often burrows beneath the earth, when it is vulgarly called a creeping root. Sometimes the internodes (137.), become much thickened, when what are called tubers are formed; or the stem lies prostrate upon the earth, emitting roots from its under side, when it is called a rhizoma, or rootstock.

132. If it distend underground, without creeping or rooting, but always retaining a round or oval figure, it is called a cormus. 133. All these forms of stem are vulgarly called roots.

134. 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.

135. The ascending axis, or stem, has nodes and internodes. 136. Nodes are the places where the leaves are expanded and the buds formed.

137. Internodes are the spaces between the nodes.

138. Whatever is produced by the evolution of a leaf-bud (142.) is a branch.

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

140. 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. (Aculei, Lat.)

V. LEAF-BUDS.

141. Buds are of two kinds, Leaf-buds and Flower-buds.

142. LEAF-BUDS (Bourgeon, Fr.) consist of rudimentary leaves

surrounding a growing vital point, the tissue of which is capable of elongation, upwards in the form of stem, and downwards in the form of wood or root.

143. FLOWER-BUDS (Bouton, Fr.) consist of rudimentary leaves surrounding a fixed point, and assuming, when fully developed, the form of floral envelopes or sexual apparatus.

144. Notwithstanding this difference, a leaf-bud sometimes indicates a tendency to become a flower-bud; and flower-buds frequently assume the characters of leaf-buds; Ex. Monstrous Pears.

145. Leaf-buds are of two kinds, the regular and the adventitious.

146. Regular Leaf-buds are only found in the axils of leaves. 147. They exist in a developed or undeveloped state in the axils of all leaves, and of all modifications of leaves.

148. Therefore, they may be expected to appear at the axils of scales of the bud, of stipules (183.), of bracts (229.), of sepals (290.), of petals (291.), of stamens (302.), and of carpels (354.); in all of which situations they are generally undeveloped; for these different organs are all modifications of leaves.

149. They are frequently not called into action, even in the axils of leaves.

150. As regular buds are only found in the axils of leaves, or of their modifications; and as branches are always the developement of buds, it follows that whatever may be the arrangement of the leaves, the same will be the disposition of the branches; and vice versa.

151. This corresponding symmetry is, however, continually destroyed by the unequal developement of the buds.

152. Leaf-buds which are formed among the tissue of plants subsequently to the developement of the stem and leaves, and without reference to the latter, are called latent, adventitious, or abnormal.

153. Adventitious Leaf-buds may be produced from any part of the horizontal medullary system, or, wherever cellular tissue is present. It has been distinctly proved, that while roots are prolongations of the vertical or woody system, leaf-buds universally originate in the horizontal or cellular system.

154. They are formed in the root, among the wood, and at the margin or on the surface of leaves.

155. They are constructed anatomically exactly as regular buds, having pith in their centre, surrounded by a medullary sheath of spiral vessels, and coated over by woody tissue and cellular integument.

156. Hence, as adventitious buds, containing spiral vessels, can be produced from parts such as the root or the wood, in which no spiral vessels previously existed, it follows that this form of tissue is either generated spontaneously, or is produced by some other tissue, in a manner unknown to us. It is most probable, that spiral vessels are spontaneous modifications of vesicles of cellular tissue, as has been before stated (34.).