548 SECONDARY CHANGES. 



it, the meristem of which remains active for along time, and, as its principal product, 

 forms a coating of cork. In most cases this formation of periderm begins simulta- 

 neously with, soon after, or even before the complete extension of the internode; in 

 the plants mentioned above, p. 535, with a long-lived epidermis, it begins later, and 

 often not for many years. 



In a large minority of the cases the initial layer of the periderm is the epidermis 

 itself: Neriuni Oleander, Viburnum Lantana, lantanoides, prunifolium, all Pomaceae 

 (Figs. 216-218), Virgilia lutea, Staphylea pinnata, Solanum Dulcamara, all investigated 

 species of Salix (Sanio), Euphorbia antiquorum ', Melastoma cymosum, and Centra- 

 denia floribundum "*. The formation of periderm in Acer striatum, which does not 

 : usually occur for many years, may also be mentioned here. In these cases only the 

 original external wall of the epidermal cells is cut off by the phellogenetic layer; 

 it is burst, and left to peel away gradually. 



In the very great majority of the plants belonging to this category the layer of 

 • cells lying immediately below the epidermis is the initial layer for the formation of 

 periderm (comp. Fig. 214, p. 528, and below, Fig. 223). The whole epidermis 

 above it is burst and thrown off. Examples : Platanus, Acer campestre, Abies pec- 

 tinata, Hakea florida; — Fagus silvatica, Rhamnus Frangula, Quercus Suber, Q. pe- 

 dunculata, Castanea, Ostrya, Carpinus, Corylus, Betula, Alnus, Ulmus, Juglans, Celtis, 

 Sambucus nigra, Plectranthus amboinensis, Crassula tetragona, Acer pseudoplatanus, 

 A. platanoides, Tilia, Catalpa, Fraxinus, Syringa, Prunus, Amygdalus, species of 

 Rhamnus, Viburnum Opulus, V. Oxycoccos, Populiis ; Medinilla farinosa, Miconia 

 •chrysoneura ', and many others. 



Closely connected with these cases is the appearance of phellogenetic divisions 

 in the second or third sub-epidermal layer, observed in Robinia pseudacacia, Gledit- 

 schia triacanthos, and Cytisus Laburnum. In this case one or two external layers 

 are thrown off, together with the epidermis, as a small bark. 



In the simplest but rarer cases only cork-cells are produced in these peridermal 

 formations, and the phellogenetic layer of meristem is reproduced by divisions in 

 purely centripetal order, phelloderm not being formed. This is the case in Nerium, 

 where the latter .seerns never to appear, and in Viburnum Lantanoides, where, accord- . 

 ing to Sanio, phejlodermal cells are at any rate still absent in stems five years old. 

 Many Coniferae also appear to form no phelloderm, though this still requires more 

 accurate investigation. In most woody plants phelloderm appears in the normal 

 course of development, whether immediately after, or almost simultaneously with, the 

 first cork-cells, or only in later stages of development, after the latter have already 

 been produced in abundance. 



The differences in these respects, which may be referred to in detail in Sanio, /.c.,are 

 partly determined by the species, while within the same species individual variations occur, 

 which often clearly depend on external causes. As regards the former, it is the rule for 

 a number of investigated species, that at first only centripetal divisions and production of 

 ' cork-cells take place, and only at a later period, often not before the second year, a 

 layer of phelloderm is formed by a reciprocal division, which is immediately succeeded 



' Schacht, Lehrb. I._p. 287. = Vochting, Bau, &c. d. Melastomeen, p. 49. 



' Vochting, /. c. 



