DEVELOPMENT OF THE VASCULAR BUNDLES. 397 



A necessary consequence of the view here propounded is that in the Monocoty- 

 ledons also every leaf-trace bundle arises from the plerome as regards that portion 

 which passes through the cylinder, but from the periblem as regards that portion 

 which runs out into the leaf. 



As regards the origination and completion of the individual bundles in the leaves 

 themselves, essentially the same rules hold good as in the case of stems. That their 

 initial strands of meristem must have a definite position and orientation, and that the 

 latter must correspond to that of the mature bundle, is obvious \ The longitudinal 

 progression of their origination and completion is guided by the direction of growth 

 of the leaf, which, as is well known, varies in the particular cases. 



That the bundles of lateral buds connected with the system of the main axis 

 originate later than the latter, scarcely requires to be mentioned ; their development, 

 or at least their completion, either proceeds centrifiigally, i.e. from the point of 

 attachment onwards into the lateral shoot, e. g. Potamogeton, Fig. 123, p. 273; or in 

 the opposite centripetal direction, e. g. in the node of Zea and Saccharum. 



Sect. 117. The development of normal lateral roots on a relative main root 

 is ' so immediately connected with the development of the vascular bundles of 

 roots, at least in the case of the Phanerogams, that it cannot be wholly passed over 

 in this place, although, strictly speaking, it belongs only in a small degree to the 

 subject of this book. 



In the Phanerogams, as shown by Nageli and Leitgeb, Reinke and Janczewski", 

 from the last of whom the following summary is principally taken, the young peri- 

 cambial layer is the chief seat of formation of normal branch-roots ; neighbouring 

 layers of cells may take part in the process to a varying extent. How far the latter 

 takes place is determined according to the particular cases to be adduced, and these 

 correspond in a great degree, though not always with accuracy, to the types of 

 differentiation of the apical meristem enumerated at p. 9, &c. 



We have here to assume as already known, that the rudiment of monopodial 

 branch-roots always arises upon the axial bundle of the relative main root, that it here 

 acquires its characteristic structure, especially the differentiation of its growing-point, 

 and then, boring through the peripheral layers of tissue, reaches the surface. 



Among the Phanerogams investigated, Janczewski distinguishes five types of 

 the processes of development in question. 



In the^rj/ of these, to which only Pistia belongs, the plerome-cylinder and the 

 periblem of the lateral root are derived from the growth and corresponding divisions 

 of the single-layered pericambium; the root-cap, or calyptrogenic layer, and the 

 epidermis, are derived from the endodermal layer. In the second type, represented by 

 Alisma Plantago, Sagittaria, and Zea, the entire lateral root, including the calyptro- 

 genic layer, arises from the pericambium, the divisions of which begin irregularly. 

 The endodermal layer only forms an exterior coating on the cap, and, in the case of 

 Zea, also the epidermis and outermost cortical layers at the base of the lateral 

 roots. With reference to the participation of the layers of the mother root, 



' Compare p. 23. 



' Nageli and Leitgeb, Entstehung u. Wachsthum d. Wurzeln, Beitr. zur Wissensch. Botanik, 

 Heft 4. — Reinke, in Hanstein's Bot. Abhandl. Heft 3 ; id. Morpholog. Abhandl. p. 1 . — Janczewski, 

 Ann. Sci. Nat. 5 ser. torn. 20, 



