396 PRIMARy ARRANGEMENT OF TISSUES. 



the same thing, the plerome strand of the main root is continued directly through 

 the hypocotyledonary axis into the bundle-ring of the stem, we can go a step 

 further, and, in agreement with van Tieghem's view*, may draw the external 

 boundary of the plerome strand close outside the external boundary of the bundle- 

 ring, even in those cases where the former cannot be distinctly traced into the apex 

 of the primary meristem. According to this view the plerome would be the axial 

 cylinder, which in the first series of cases only becomes marked off from the periblem 

 by the differentiation of the initial cylinder in its external part, and which in the other 

 series of cases is continued with a sharp boundary into the extreme apex of the stem, 

 where it shows no differentiation other than the primary layers of meristem. Among 

 the Angiosperms it appears to be chiefly plants with a very flat apical meristem, 

 forming new internodes at definite intervals, which belong to the first category, while 

 the second includes those with a more elongated apex. With the latter are further 

 connected the elongated apices of plants with one axial vascular bundle, which 

 arises from a sharply defined plerome-bundle. 



Medullary bundles situated inside a distinct ring require, after what has been said, , 

 no further explanation with reference to their origin from the primary rows of meristem. 

 For the net of bundles in the stem of Gunnera the same holds good, according to 

 Reinke '■', as for the ring of typical Dicotyledons. The Nymphaeacese and Auriculas 

 require still more accurate investigation with reference to the question under discussion. 



For the Monocotyledons ' the question before us may be generally answered by the 

 statement that the cylinder containing the bundles, defined in this sense above at 

 p. 261, is derived from the plerome strand. This is often to be traced into the 

 extreme apical meristem, e. g. Grasses, Polygonatum, Canna, Potambgeton, Trades- 

 cantia sp., Asparagus '; or it may only be differentiated below this, e. g. Epipactis, &c., 

 according to Falkenberg. The outer layer of the bundle-cylinder, in which the lower 

 ends of the bundles lie, falls in the outer boundary of the plerome, or lies within it. 

 The succession of development of the bundles follows the general rule here also. 

 In consequence of this, and of the course of the bundles described above, we find 

 them, in the case of the Palm-type, appearing in centrifugal order as seen in suc- 

 cessive cross-sections of young internodes, the median bundles which penetrate most 

 deeply appearing first, and so on. The completion of the tissue surrounding the 

 bundles, especially the cessation of longitudinal divisions, and the increase in volume 

 of the meristematic cells which become converted into parenchyma, proceed in the 

 same order ; in the external region, partly for the reasons already given, relatively 

 more abundant longitudinal divisions and smaller growth of the elements in the 

 transverse directions take place. As long as the longitudinal divisions, which cease 

 in centrifugal order, persist, the periphery of the cylinder is occupied by a meristematic 

 narrow-celled ring, which Sanio identifies with his thickening-ring demonstrated in 

 the case of the Dicotyledons, a view which is so far, but only so far correct, as both 

 constitute that zone of the plerome body which is engaged in differentiation and in 

 the formation of vascular bundles. 



' Ann. Sci. Nat. 5 ser. torn. XVI. p. 112, note. » Morpholog. Abhandl. p. 67. 



' [Compare Guillaud, Ann. Sci. Nat. 6 ser. Bot. torn. V. pp. 1-176.] 

 • Hanstein, I.e. — Falkenberg, I.e. 



