208 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 35. 



The secondary extra-stelar tissue, is formed by a layer of meris- 

 matic cells, which is known as the Phellogen. 



In the stem the place of origin of the phellogen is by no means 

 uniform. It may be stelar or extra-stelar: it is sometimes formed 

 by the epidermis becoming merismatic (e.g. Pomese, Salix, Vibur- 

 num Lantana, Jasminum, Nerium Oleander, Aucuba, Euonymus, 

 Solanum, etc.) ; most commonly it is the hypodermal layer of 

 cells, the outermost layer of the cortex, which becomes merismatic 

 and constitutes the phellogen (e.g. Platanus, Acer, Fagus, 

 Quercus, Castanea, Betula, Alnus, Ulmus, Populus, Ailanthus, 

 Abies pectinata, etc.) ; in some cases (e.g. Robinia Pseudacacia, 

 Gleditsclda triacantlws , Cytisus Laburnum), it is the second or third 

 layer of the cortical parenchyma, reckoned inwards from the 

 epidermis, which becomes the phellogen : in other cases the 

 phellogen is formed at a greater depth from the surface, being 

 developed from a more internal layer of cells of the cortex, 

 sometimes even from the endodermis (e.g. Coffea arabica ; sub- 

 terranean shoots of some Leguminosae such as Lotus corniculatus, 

 Trifolium alpestre) ; or, finally, it is stelar, being formed from a 

 layer of cells belonging to the pericycle (e.g. Myrtaceae, Onagracea?, 

 Hypericacese, Ericaceae, most Caryophyllaceoe, Lonicereas, Vitis, 

 Clematis, Berberis, Rosa, Spiroaa, Ribes, etc.) 



The development of tissue from the phellogen follows the same 

 law as in the case of the cambium (see p. 193). Generally speak- 

 ing, a tissue, the periderm, is formed on the outer side of the 

 phellogen by repeated centripetal division ; whilst on the inside 

 of the phellogen a tissue, the phelloderm, is formed by repeated 

 centrifugal division. The periderm constitutes the secondary 

 tegumentary tissue of the stem or root ; the phelloderm constitutes 

 the secondary cortex. The developmental relations between the 

 two tissues are not constant. In some cases the formation of 

 phelloderm only begins after a considerable mass of periderm has 

 already been produced ; but in others, the formation of the two 

 tissues goes on almost simultaneously. The relation between the 

 amount of periderm and the amount of phelloderm formed by one 

 and the same phellogen is by no means constant, and may be gener- 

 ally stated thus : the more superficial the phellogen, the greater 

 the relative amount of periderm ; and further, a larger propor- 

 tion of phelloderm is formed in subterranean than in subaerial 

 stems. Hence whilst the development of periderm is most marked 

 in subaerial stems with superficial phellogen, there is little or no 



