35.] CHAPTER II. THE TISSUES. 209 



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. 



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 

 epiblema ; more commonly from the exodermis, or from the next 

 internal layer of the primary cortex, as in those Pteridophyta 

 (Marattiaceae, Ophioglosseee) and Monocotyledons (aerial roots of 

 Philodendron, Monstera, Imantophyllum, etc. ; terrestrial roots of 

 Iris, Agave, Asphodelus) in which any formation of periderm takes 

 place ; as also in a few woody Dicotyledons (e.g. Artanthe, Clusia, 

 E/uyschia, Jasminum) in which the formation of secondary vas- 

 cular tissue takes place relatively late ; and in the Cycads among 

 Gymnosperms. In the great majority of Dicotyledons and Gym- 

 nosperms 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 phello- 

 gen, 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. 

 Faba vulgaris, Alchemilla, vulgaris, Gaillardia aristata), where 

 the primary cortex persists (see p. 207), only phelloderm is de- 

 veloped. 



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 phel- 

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

 the primary phellogen has passed over into some form of perma- 

 nent 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 

 v. s. B. P 



