35.] CHAPTER II. THE TISSUES. 205 



subsequent growth in thickness is effected by successive secondary cambium- 

 rings developing rings of secondary vascular bundles ; these secondary cam- 

 bium-rings may be developed: (a) in the pericycle ; stem of Chenopodiaceae, 

 Amarantacete, Phytolacca, Nyctaginacese, and of some Gnetums, Cycas, and 

 Encepbalartos among Gymnosperms : (I) in the primary cortex ; stems of 

 some Menispermacese (e.g. Cocculns laurifulius, Cissampelos Pareird) : (c) in 

 the secondary bast ; stem of Glycine (Wistaria) : (d) in the secondary wood ; 

 twining stems of Bauhinia, and some Bignoniaceae and Malpighiacese : (e) in 

 the secondary cortex (phelloderm), derived from the pericycle; root of Cheno- 

 podiacese, Amarantaceoa, Nyctaginacese. In some cases, however (e.g. roots of 

 many Convolvulaceae), secondary cambium-layers are formed in the secondary 

 cortex (phelloderm), whilst the primary cambium-layer still remains active. 



4. The cambium-layer is normal, but it does not produce equal amounts of 

 wood, or of bast, or of both wood and bast, at all points of its circumference, 

 as is normally the case, with the result that the cambium-layer is not a circle 

 in transverse section, but is very irregular and undulated in form, the wood 

 and the bast dovetailing into one another, as it were, by their respective 

 projecting thicker portions, (a) The development of bast is uniform, that of 

 the wood uneven ; stems of various species of Cissus, Piper, Bauhinia 

 (Caulotretus heteropliyllus) ; root of Ononis spinosa ; (&) the development of 

 neither wood nor bast is uniform, but at the points at which the development of 

 the bast is less active, that of the wood is more active ; twining stems of many 

 Bignoniaceae (Bignonia, Callichlamys), of Phytocrene (Olacineae), and of some 

 Malpighiacese (Banisteria, Tetrapterys), Apocynaceae (Condylocarpon, Echites). 



An interesting modification of this peculiar mode of development occurs in 

 the stem of species of Strychnos ; at certain regions the cambium-layer pro- 

 duces thick masses of secondary bast which project into the wood ; after a 

 time the cambium of these regions ceases to act, whilst a new segment of 

 secondary cambium is formed in the pericycle across the projecting masses of 

 bast at the level of the rest of the cambium-layer ; thus the cambium-ring is 

 reconstructed, forming wood centrally and bast peripherally, with the result 

 tbat the masses of bast mentioned above become covered peripherally by 

 a layer of wood, constituting in fact isolated groups of bast, termed Phloem- 

 islands^ surrounded by wood. 



5. There is no primary cambium-layer, the bundles being all closed ; second- 

 ary growth in thickness is effected by a ring of meristem quite external to 

 the primary bundles ; this occurs in the stems and roots of Monocotyledons 

 (arborescent Liliaceae, such as Yucca and Dracaena ; and some shrubby Iridaceae, 

 such as Aristea) ; the ring of meiistem is usuaUy developed in the pericycle, 

 but in the roots of Dracaena it is formed partly from the pericycle and partly 

 from the cortex. This meristem-ring is not termed a cambium-ring, because 

 it does not form wood on one side, and bast on the other, but it forms, centri- 

 fugally, entire closed concentric (with external wood) bundles, together with 

 intervening fundamental tissue. 



6. There is no proper cambium-layer, but the primary bundles are invested 

 by a pericyclic meristem-ring, which gives rise externally to a considerable 

 amount of parenchymatous secondary cortex, and internally to a small amount 

 of vascular tissue : stem of Isoetes. 



