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



to the vascular tissues, but takes place wherever a young develop- 

 ing cell grows more actively, in any dimension, than the cells with 

 which it is at first in contact ; a notable example is the growth of 

 the laticiferous coenocytes of Euphorbia (see p. 141). 



Whilst undergoing these changes of form, the desmogen-cells 

 undergo, as already indicated, changes in the structure and 

 chemical composition of their cell^walls in accordance with the 

 particular kind of tissue to which they are to give rise; and, in 

 some cases (tracheae, tracheids, fibres) they lose 1 their protoplas- 

 mic cell-contents ; the walls become more or less thickened (spiral, 

 annular, reticulate, in. primary wood) and pitted (with simple 

 pits ; or circular bordered pits ; or oval bordered pits, either small 

 and numerous, or large extendirg across a whole face of the wall, 

 giving it a scalariform appearance, see p. 104) ; and then the 

 absorption, more or less complete, of the septa takes place, which 

 leads to the formation of the vessels. 



Glandular tissue is frequently developed in the secondary wood and bast, in 

 the form, sometimes, of saes containing crystals, in the parenchyma (including 

 medullary rays) of the wood (e.g. Vitis, and some leguminous trees) or more 

 commonly in that of the bast : of resin-ducts which occur in the secondary wood 

 of certain Abietineae, running horizontally in the medullary rays and vertically 

 in the wood, but rarely found in the secondary bast, whereas in other plants 

 which possess these structures, they are rare in the wood- but abundant in the 

 bast (e.g. Terebinthaceae, etc.-) : of laticiferous vessels, rare in the wood (except 

 the Papayaceas, where the wood consists largely of parenchyma), abundant in 

 the bast. 



The foregoing is an account of the development of secondary vascular and 

 conjunctive tissue, as it takes place in the great majority of Gymnosperms and 

 Dicotyledons : but this is bv no' means the only mode in which this develop- 

 ment takes place. The following are the more remarkable deviations from the 

 mode already described : 



1. There is a normal cambium-Ting, but an additional layer of meristem is 

 formed from pith-cells on the inner side of the ring of vascular bundles, produc- 

 ing secondary wood peripherally, and secondary bast centrally ; stem of Tecoma 

 (Bignonia) radicans, and of species of Acanthus and Campanula, some Apo- 

 cyuaceffi (Apocynum cannalnnuni) , Periploca grceca, Acantholimon ghunaceum. 



2. There are several distinct eambium-rings, each producing a solid cylinder 

 of wood and bast : stems of climbing Sapindacese (e.g. Serjania, Faullinia). The 

 cause of this peculiar structure is that the primary bundles are arranged, not in 

 a simple ring, but irregularly, at very different distances from the surface ; 

 hence, when the interfascicular cambium is developed, it does not connect all 

 the bundles together, but separate groups of them, generally a larger central 

 group and several smaller peripheral groups, each with its own distinct cambium- 

 ring. 



3. The normal cambium-layer has only a limited period of activity ; the 



