SECONDARY CHANGES OUTSIDE THE ZONE OF THICKENING. 547 



appears also to take place on the formation of the first layer of cork in Melaleuca 

 styphelioides. 



As will be shown in greater detail below, the duration and productiveness of a 

 corkmeristem, when once formed, are remarkably various in the special cases. 

 It may remain active for decades of years, and longer, and produce masses of tissue ; 

 and, on the other hand, its generative activity may come to a standstill after a few 

 divisions, and it may itself, together with the layers it has produced, become con- 

 verted into a permanent form of tissue. 



It may happen exceptionally, and in no known case as a phenomenon of normal 

 development, that in layers of meristem with persistent growth the cell which typically 

 remains meristematic becomes a cork-cell incapable of division. The meristematic 

 properties and functions are then transferred to the next inner parenchymatous cell. 



The origin of the two forms of tissue derived from the phellogenetic meristem, 

 when the latter remains permanently active, is determined in such a manner, that all 

 cells formed on its outer side become cork-cells, while those formed on its inner side 

 become phelloderm. Thus purely centripetal phellogens only form cork-cells, while 

 reciprocal ones form the latter on the one side, and phelloderm on the other. Per- 

 manently active, purely centrifugal meristems, if they existed, would accordingly only 

 form phelloderm. The occurrence of the purely centrifugal course of division is, 

 however, limited to cases in which activity soon ceases, and in these cases either the 

 inner of the few layers produced become phelloderm, and the outer cork-cells (Loni- 

 cera Caprifolium), or they are all converted into cork-cells. 



The cells of the meristematic layer generally show the structure indicated by 

 this term ; individual peculiarities, e. g. the presence of chlorophyll in Sambucus 

 nigra, and lateral thickening of the walls in Salix, &c., scarcely need to be mentioned 

 here. Their form is that of polygonal plates, the transverse and longitudinal sections 

 being more or less sharply quadrangular, and the radial diameter, as a rule, con- 

 siderably shorter than the others. 



The properties of the phellogenic cortical parenchyma, or phelloderm, are in all 

 essential points similar to those of the outer dense parenchyma of the cortex ; like 

 the latter, it shows the phenomena of dilatation and sclerosis. The one generally 

 applicable distinction from the latter consists in its origin as a supplementary struc- 

 ture added subsequently by the phellogenetic meristem, and in its radial arrangement 

 resulting from its mode of origin. 



The structure of the cork-cells has been dealt with in Sect. 24, p. 108. 



The changes which are produced by the phellogenetic formations in the whole 

 primary and secondary cortex depend upon their place of origin and special character. 

 Three principal phenomena are accordingly to be distinguished, namely ih^ formation 

 of superficial periderm, of internal periderm and bark, and of Lenticels. It follows 

 from the nature of the subject that these three kinds of structure are nearly related Jo 

 one another, and therefore that transitions may also appear between them. 



Sect. 175. Superficial periderm. In most stems of woody plants, in tubers, and 

 also in some few roots, as those of Anisostichus (Bignonia) capreolata, and the 

 Clusiaceae \ the epidermis is replaced by a periderm arising within or close beneath 



' Van Tieghem, Ann. Sci. Nat., 5 ser. torn. XIII. p. 258. 



N n 2 



