150 PART II. ANATOMY AND HISTOLOGY. [ 35 



8\st also in some roots (e.g. Vicia Faba, Alchemilla vulgar is), in 

 which the development of secondary vascular tissue is not very 

 active. The extension of the tissue is effected by tangential 

 growth and radial division of the cells. 



The secondary tegumentary and cortical tissue is formed by 

 a layer of merismatic cells, which is known as the Phellogen. 



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

 uniform. It is sometimes formed by the epidermis becoming mer- 

 ismatic (e.g. Pomese, Salix, Viburnum Lantanctj Euonyrnus, So- 

 larium, 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 pec- 

 tinata, etc.) : 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 endodennis 

 (e.g. Cojfea ctrabica ; subterranean shoots of some LeguminosaB 

 such as Lotus corniculcttus, Trifolium alpestre) ; or, finally, it is 

 stelar, being formed from a layer of cells belonging to the peri- 

 cycle (e.g. Hypericum, Erica, most Caryophyllacese, Lonicera, Vitis, 

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



The development of tissue from the phellogen follows the same 

 law as in the case of the cambium. Generally speaking, a 

 tissue, the periderm, is formed on the outer side of the phel- 

 logen by repeated centripetal division ; whilst on the inside of 

 the phellogen a tissue, the phettoderm, is formed by repeated 

 centrifugal division. The periderm constitutes the secondary 

 tegumcntaiy tissue of the stem or root ; the phelloderm consti- 

 tutes 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 uniform : whilst 

 the development of periderm is most marked in subaerial stems 

 with superficial phellogen, there is little or no phelloderm in 

 these stems ; again, in subaerial stems with a deeply-placed (e.g. 

 pericyclic) phellogen, periderm and phelloderm are developed about 

 equally ; finally, in subterranean stems with a pericyclic phellogen, 

 the well-developed phelloderm may exceed the periderm. 



