THK VEGETABLE ORGANIZATION. 297 



these little vesicular terminations of the fibrils of the roots that 

 plants obtain tlieir nourishment from the soil in which they 

 grow. They are found equally in the lofty forest tree and the 

 herb which grows beneath its shade, and are equally necessary 

 to the existence of both ; for if by accident or design the fibrils 

 are materially injured, the plant withers and dies. In their in- 

 ternal structure, roots, with some exceptions, will be found to 

 resemble the stem; in fact, in many instances, they consist of a 

 main descending trunk, branches, and twigs, the organization 

 of which is precisely conformable to the ascending stem and its 

 divisions, as it is found above ground. 



The stem or trunk varies much in different families of plants. 

 In those among the acotyledonous plants in which it occurs it 

 consists of a thin epidermis or cuticle, surrounding a pulpy 

 matter, the texture of both which parts is almost entirely cel- 

 lular ; a few fibres or vessels are, however, found in the stem of 

 the fern tribe, forming an exception to the general internal 

 organization of this division of the vegetable kingdom. In the 

 monocotyledonous plants, the stem consists also of a cuticular 

 envelope and internal cellular substance, but the cuticular 

 portion is more dense and firm, and the cellular texture of the 

 internal part of the stem is traversed by longitudinal fibres or 

 vessels, several of which are arranged together in bundles. In 

 the dicotyledons, the stem is composed of bark, wood, and pith. 

 The bark, which corresponds to the cuticular part of the stems 

 of the acotyledonous and monocotyledonous plants, is formed of 

 several layers of cellular and fibrous texture; the wood or fibrous 

 portion of cellular texture, and closely compacted vessels tra- 

 versing the cells in a longitudinal direction, and symmetrically 

 arranged so as to form concentric and divergent layers ; and the 

 pith, which occupies the central portion of the stem, of a light 

 spongy cellular tissue. 



The development of this apparently intricate organization 

 from the simple elements of cell and vessel, according to the 

 principles already laid down, may be conceived to take place 

 in the following manner. Let us suppose a complete circular 

 range of secondary vesicles becoming dilated by the process 

 of growth in the interior of a primary or generating cell. 

 It is evident that the envelope of this cell, pressed at all 

 points of the circumference by similar parts which are growing 

 at once in length and breadth, will take the form of a cylinder; 

 but it is also evident that the secondary vesicles will be more 

 compressed towards the centre than at the circumference, and 

 that, consequently, a transverse section will exhibit each of these 

 secondary cells under the form of a wedge. If the eight 

 vesicles grow in a longitudinal direction, without giving rise 

 to other vesicles within them, we shall then have an organi- 

 zation similar to that of the long stalk which supports the flower 

 of the Nijmphaa alba, or white water lily, a monocotyledonous 

 plant. Each of these vesicles will form an empty canal, of 



