OF BOTANY. XVU 



51. The compound organs are the curia (52) and its appendages (158). 

 53. The axis may be compared to the vertebral column of animals. 



53. It is formed by the developement of an embryo, or of a Leaf-bud. 



54. An embryo is a young plant, produced by the agency of the stamina and pistil.-, 

 and developed within a seed. 



55. A leaf-bud is a young plant, produced without the agency of the stamina and 

 pistils, and either enclosed within rudimentary leaves called scales, or naked. 



56. Seeds propagate the species. 



57. Leaf-buds propagate the individual. 



58. All the phenomena connected with the growth of plants arc caused by an in- 

 herent vital action. 



59. When the vital action of a seed or bud is excited, the tissue devclopes in 

 three directions, the one upwards, the other downwards, and the third horizontal. 



60. That part which developes downwards is called the descending axis or root ; 

 that upwards, the ascending axis or stem ; and the part from which these two a.\c» 

 start is called the collet or neck. 



61. This elongation in opposite directions takes place simultaneously: hence it 

 follows that all plants must necessarily have an ascending and descending axis, or 

 a stem and root. 



62. The only apparent exceptions to this are vesicular Alga?. 



III. ROOT. 



63. The root is formed by the descending and dividing fibres of the stem. 



64. Anatomically it differs from the stem in the absence of spiral vessels (~3), of 

 pith (15), and of buds, and in the want of stomata (11). 



65. The functions of the root are to fix plants in the earth, and to absorb nu- 

 triment from it. 



66. This absorption takes place almost exclusively by the extremities, which 

 consist of a lax coating of cellular tissue lying on a concentric layer of woody fibre, 

 in the midst of which is placed a bundle of ducts. 



IV. STEM. 



67. The stem is produced by the successive developement of leaf-buds (142), 

 which elongate in opposite directions. 



68. If an annular incision be made below a branch of an Exogenous planl 



the upper lip of the wound heals rapidly, the lower lip does not : the pari above tlie 

 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 liga- 

 ture swells, that below it docs not swell. 



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

 meter descends. 



71. If a growing branch is cut through below a leaf-bud, that branch nevei 

 creases in diameter between the section and the first bud below it. 



72. The diameter of all Exogenous stems increases in proportion to the num 

 of leaf-buds that are developed. 



73. The greater the number of leaf-buds above a given part, the greater tin 

 diameter of that part ; and vice versa. 



74. In the spring the newly forming wood is to be traced to the form of fibres de- 

 scending 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 Exi 

 nous plants increase in diameter, proceeds from the leaf-buds. 



76. Their elongation upwards gives rise to new axes, with the appendaj 



same; their elongation downwards increases the diameter of that part of the axis 

 which pre-existed, and produces roots. 



77. Hence, while the stem is formed by the successive evolution of leaf-buds, the 

 root, which is the effectofthat evolution", lias i" 1 1 laf-buds. 



78. The leaf-buds thus successively de\. firmly connected by the cellu- 

 lar tissue of the stem, which proceeds from th< m\ ards, or from the circumfe- 

 rence to the centre. 



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



80. In vascular plants il is eithei formed by successive additions to the outsid 

 the wood, wken it is called Exogenous; or by successive additions lo il • • entre, 



