35] CHAPTER II. THE TISSUES. 151 



In the root, as in the stem, the position of the phellogen, and the 

 products of its activity, are various. The phellogen is developed 

 but rarely (e.g. Solidago) from the epiblem ; more commonly from 

 the exodermis, or from the next internal layer of the primary 

 cortex, as in a few woody Dicotyledons (e.g. Jasminum) in which 

 the formation of secondary vascular tissue takes place relatively 

 late ; and in the Cycads among Gymnosperms. In the great ma- 

 jority of Dicotyledons and Gymnosperms the phellogen of the root 

 is stelar in origin, being derived from the pericycle. 



As in the stem, so in the root, the phelloderm is more highly 

 developed when the phellogen is deeply placed than when it is 

 superficial ; but even with a uniform position of the phellogen, the 

 relation between the periderm and the phelloderm developed, varies 

 considerably : thus, among plants with a pericyclic phellogen, 

 whilst the development of periderm and phelloderm is sometimes 

 about equal (e.g. Willow), no phelloderm but only periderm is 

 developed in Nerium, whilst in some others (e.g. Vicia Faba, 

 Alcliemilla vulgaris), where the primary cortex persists (see 

 p. 149), only phelloderm is developed. 



It frequently happens in both stems and roots that the first- 

 formed primary phellogen has but a limited period of merismatic 

 activity ; this is always the case when the primary phellogen is of 

 deep origin (pericyclic in roots), whereas when it is of superficial 

 origin (e.g. epidermal or hypodermal phellogen in stem of Beech, 

 Hornbeam, Silver Fir, Cork-Oak, Cork -Elm), the primary phellogen 

 is frequently persistent. In the former case, however, when the 

 primary phellogen has passed over into some form of permanent 

 tissue, a new secondary phellogen, also of limited duration, is 

 developed internally to the first, and this process is repeated at 

 intervals ; hence the phellogen-layers become successively more and 

 more deeply seated, penetrating at length into the bast-tissue of the 

 stele. 



The periderm, or secondary tegumentary tissue, the tissue formed 

 externally from the phellogen, consists of parenchymatous cells 

 more or less cubical in form, though sometimes somewhat elongated 

 tangentially (Fig. 118) ; the cell-walls may be thin or considerably 

 thickened ; generally speaking, the walls are completely suberised 

 (see p. 76), whence the tissue is often termed Cork; the cells 

 gradually lose their protoplasmic contents, and become filled with 

 air ; moreover, no intercellular spaces are formed in the tissue. 



In view of its structure, it is clear that the periderm is a tissue 



