530 



PHYSIOLOGY. 



leaves, but not possessed of a cambium-region ; while the outer circle is 

 of the ordinary open or unlimited character. In the Sapindacese, Malpi- 

 ghiaceae, and some other Orders, part of the fibre-vascular bundles remain 

 separate from the principal circle, and lay the foundation of a number of 

 secondary cylinders of wood enclosed by a common bark ; this pheno- 

 menon may'be well observed in Calycanthus, where a square form of the 

 stem results from four fibre-vascular bun dies remaining free from the central 

 cylinder of wood in this way. In the Nymphseaceas we find a very aber- 

 rant condition : the fibro-vascular bundles, formed of vessels and paren- 

 chymatous cells alone, without wcod, are quite isolated, destitute of cam- 

 bium, and form a complicated interlacement closely resembling that oc- 

 curring in Monocotyledons there being no distinction of pith and medul- 

 lary rays, and no bark. 



Dicotyledonous fibro-vascular bundle (Plane-tree) of one year's growth. A. Transverse 

 section. B. Vertical section ; s I, suberous layer of the bark ; c p, cortical parenchyma ; 

 I, liber ; c, cambium-region ; d, ducts lying in the prosenchyma, or wood-cells, pr ; m s, 

 medullary sheath of spiral vessels ; p,pith. The structures connected by b belong to the 

 bark, those marked w to the wood. In A, the bundle is seen to be bounded on each side 

 by a medullary ray, running from the pith to the cortical parenchyma. Magn. 50 diam. 



- A still more frequent source of diversity lies in the varied nature and 

 mode of arrangement of the elements of t\z wood. In the Plane (fig. 578) 

 we see the spiral and annular vessels succeeded by a body of prosenchyma, 

 in which are scattered large pitted ducts. In the Hazel and Alder these 

 ducts are far more numerous, as they are also in the Lime. In the Oak 

 the prosenchymatous cells are very small, and become greatly thickened, 

 but the ducts are large. The Box has very small and dense prosenchyma- 

 cells and few and small ducts. In the spongy wood of the Bombacea3 the 

 prosenchyma is almost wholly replaced by thin- walled parenchyma. 



