SSCONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 545 



the latter and their meristem with them under one general name, and to choose 

 Mohl's old term Periderm, with a' modification of its original meaning, comp. p. 114. 

 The peridermal structures always arise in a layer of cells which has already 

 been differentiated. This is their initial layer, and is either fdirmed by the single-layered 

 epidermis, or by a single sub-epidermal layer of parenchymatous cells, which may 

 occur at various depths, and runs approximately parallel to the surface of the part. 

 The periderm consists of the phellogenetic meristem, and the tissues that have been 

 derived froni it, which, in all cases, include a single or many-layered stratum of 

 Cork-cells or Cork-tissue, to which usually, but not s\-via,js, phellogem'e 01 peridermal 

 parenchyma, the Phelloderm of Sanio, is added. 



If a layer of cork is formed in the interior of a mass of tissue, the tissue lying 

 outside it dries up, and is eventually thrown off as lark (Rhytidoma, Mohl). The 

 formation of bark is the immediate consequence of the internal formation of periderm, 

 and the name is as a rule employed for the dried-up tissues and the adjacent peri- 

 dermal layers conjointly; with the formation of periderm that of the lenticels stands in 

 the closest connection, but the consideration of the latter, on account of certain 

 peculiarities, will be omitted here, and deferred to Sect. 179. 



The general course of the formation of periderm has to a great extent already 

 been stated in Sect. 24. The initial layer of cells becomes converted by tangential 

 divisions into a multiseriate zone, the elements of which partly remain meristematic, 

 and partly pass over into tissue. So far, and so long as the former is the case, they 

 have the capacity of following the dilatational growth by means of their increase in 

 size, and successive radial divisions occur in them, as in the cells of the dilated 

 parenchyma, by means of which the original average breadth is always again 

 approximately restored. Accordingly, all cells' belonging to the periderm are ar- 

 ranged in radial rows, each of which originally corresponds to one of the initial cells, 

 and may become successively doubled, and they further form concentric (tangential) 

 layers, see Fig. 216. 



In the course of the tangential divisions in an initial cell and the radial row 



derived from it, two extreme forms may in the first instance be distinguished, which, 



after Sanio, are termed the centripetal and centrifugal forms ; besides these, there is 



' the mode of succession which proceeds alternately in the centrifugal and centripetal 



directions, and may be generally termed reciprocal. 



In the centripetal succession the initial cell is divided into two daughter-cells, 

 the outer becomes a tissue-cell, the inner remains meristematic, and continues the 

 same process in such a manner that, in the further successive bipartitions, the inner 

 cell always remains meristematic, while the outer becomes tissue. 



In the second case the order is reversed. Of the products of the successive 

 tangential divisions the outer cell always remains meristematic, while the inner becomes 

 an element of the tissue. 



In the reciprocal succession (Figs. 216-218) the division begins in one of the 

 orders mentioned, then changes over into the other, and may then again change back 

 to the first. At the first origin of the phellogenetic layers Sanio found the following 

 cases of reciprocal succession : — 



I. The first two divisions in centripetal order, then the innermost cell becomes 

 a tissue-cell, the second from the inside becoming a meristematic cell ; after the third 



N n 



