THE STEM. 



25 



of condensed, fibrous tissue, and proceeding from 

 the central pith to the liber or inner bark. At c c, 

 are seen other lines of concentrated fibrous tissue, 

 forming part of concentric circles, which com- 

 mence near tlie centre, and follow each other at 

 intervals to the circumference. These are the 

 annular layers, and mark out the growth of each 

 successive year. A few large openings, or la- 

 cunte, are seen interspersed through the general 

 structure of hexagonal cells. Fig. 3, is a verti- 

 cal section of the oak, affording a specimen of 

 the true dicotyledonous class. Here there is the 

 central pith a, medullary layers b b, the an- 

 nular layers c e, the alburnum d, liber e, and 

 epidermis/. The small hexagonal cells are also 

 here present ; but a number of larger oval open- 

 ings are also irregularly dispersed through the 

 whole. Figs. 4, 6, 6, are vertical sections of 

 the fir ; fig. 4, is a vertical section made at right 

 angles to the medullary rays, and exhibits a 

 number of oval disks, or lateral sections of the 

 celhilar tubes ; fig. 5, is a section of the same 

 tree, made parallel to the medullary rays. Fig. 

 7, is a transverse section of the sugar cane, highly 

 magnified ; fig. 8, a transverse section of the oak; 

 fig. 9, a transverse section of the elm. 



Now that the internal structure of the different 

 kinds of stem is known, it will be more easy for 

 us to examine that which the roots present. 

 The roots are generally organized like the stems. 

 Thus, in dicotyledonous trees, a transverse section 

 of the roots presents concentric zones of wood 

 disposed in a circular form, and enclosed one 

 within the other. It has been said that the best 

 distinction between the stem and the root, is 

 found in the circumstance that the latter is des- 

 titute of a medullary canal ; while, on the con- 

 trary, it is known that it always exists in dico- 

 tyledonous trees. From this it necessarily fol- 

 lows, that the medullary insertions are also 

 wanting in the roots. 



This difference, however, appears of little im- 

 portance, and even entirely at variance with 

 facts. Indeed, it wiU be found, in a great num- 

 ber of vegetables, that the medullary canal of 

 the stem is prolonged without any interruption, 

 into the body of the root. If the stem and root 

 of a horse-chestnut, of two years old, be split 

 in the longitudinal direction, the medullary canal 

 of the stem wiU be seen extending to the lowest 

 part of the root. We find the same appearances 

 in the young plant of the sycamore or maple. 

 But very frequently, the medullary canal, which 

 was very distinct in the plant soon after germin- 

 ation, gradually diminishes, and at last disap- 

 pears as vegetation goes on ; so that, in the 

 root of the adult plant, it is no longer to be seen. 

 Consequently we cannot assume as a distinctive 

 anatomical character between the stem and the 

 root, the want of a medullary canal in the latter, 

 since it almost always exists in the radicle of 



the germinating seed, and often in the root of 

 many vegetables, long after the first period of 

 their life. The tapering roots, however, even 

 those which are the largest, never present it in 

 their ramifications. 



Until lately, the want of tracheae in the root 

 had been considered as affording a distinctive 

 character between the anatomical structure of 

 the root and that of the stem ; but two of the 

 German naturalists who have made the most 

 important observations in vegetable anatomy. 

 Link and Treviranus, have found these vessels 

 in the root of certain plants ; and still more re- 

 cently, M. Amici has um-oUed tracheae in the 

 roots of several. 



The difference which we have seen to exist 

 in the organization of the trunk of the dicoty- 

 ledones, and of the stipe of the monocotyledones, 

 is equally observed in their roots. In fact, in 

 the monocotyledones, a vertical root is never 

 found forming a continuation of the stem. This 

 disposition is a consequence of the mode in which 

 the seed is developed at the period of germina- 

 'tion, since, as we shall see more particularly 

 when we speak of that function, the central and 

 principal radicle is always destroyed soon after 

 germination. 



There is another very remarkable difference 

 between roots and stems. The latter, in general, 

 grow in height by every portion of their extent, 

 whUe the roots lengthen at their extremity 

 only. This was demonstrated by Du Hamel'a 

 experiments. If little marks, at the distance 

 of an inch, are made in a young stem, at the 

 moment of its developement, it will be seen, 

 when the growth is terminated, that the spaces 

 between these marks have been greatly enlarged. 

 If the experiment be repeated on the roots, it 

 will be found that the spaces remain unaltered, 

 whUe the root itself has been lengthened, which 

 proves that the increase in length has taken 

 place by its extremity only. 



The branches, in their mode of growth, ex- 

 hibit nearly the same appearances as the trunk 

 from which they proceed. They originate in a 

 bud, and form also a cone that consists of pith, 

 wood, and bark, or rather, they form a double 

 cone. For the insertion of the branch into the 

 trunk resembles also a cone, whose base is at the 

 circumference, and whose apex is at the centre ; 

 at least, if it is formed in the first year of the 

 plant's growth, on the shoot of the present year ; 

 hut falling short of the centre in proportion to 

 the lateness of its formation, and number of in- 

 tervening layers. Like the trunk and root, it 

 increases also in width by the accession of new 

 layers, and in length ])y the addition of new 

 shoots, at least in as much as regards its external 

 portion ; exhibiting, however, some slight pe- 

 culiarities, in as far as regards its insertion. The 

 apex being never carried nearer to the centre 



