528 



PHYSIOLOGY. 



the rind outward. The original 

 central fibro-vascular system of 

 the stem remains unaltered. 



The region, at the junction of 

 the central and cortical paren- 

 chymas, where the fibro-vascular 

 bundles terminate should per- 

 haps be called a oxm&um-region. 

 since the cellular tissue situated 

 here retains its developmental 

 power in many cases. The es- 

 sential difference between this 

 and the cambium-ring of Dico- 

 tyledons depends on the fact of 

 its not coinciding, in a parallel 

 arrangement, with the cambium- 

 region of the tibro-vascular bun- 

 dles, but with the extremities of 

 the bundles, which always re- 

 main isolated from each other. 

 The successive layers of fibrous 

 structure in Draccena &c. are 

 formed in like manner of isolated 

 bundles imbedded in parenchy- 

 ma ; they are unconnected with 

 the old bundles of the primary 

 axis, but are continuous above 

 with the lower ends of bundles be- 

 longing to the branches occurring 

 in these stems. 



The stems of herbaceous Mo- 

 nocotyledons have the fibro-vas- 

 cular system always in the form 



Fig. 576. 



Monocotyledonous flbro-vascular bundle (from 

 the spadix of Phoenix dactylifera). A. Trans- 

 verse section. B. Vertical section; p, parenchy- 

 ma in which the bundles lie ; tc, wood-cells ; 

 s v, spiral vessels ; d, reticulated ducts ; v p, vasa 

 propria; I, liber-cells. Magn. 100 diam. 



of "stringy" fibres imbedded in 



succulent parenchyma ; and in those perennial stems of the Class which 

 acquire a solid woody structure the ligneous character depends, not on 

 the fibre-vascular system, but on the general parenchyma of the stem 

 having its cells lignified (sclerenchyrna), of which we have examples in the 

 Cocoa-nut and other Palms, in the Bamboo, &c. 



The rind of the Monocotyledonous stem, totally different from true bark, 

 is generally little developed. On herbaceous stems it is a mere epidermis ; 

 but on fleshy rhizomes it sometimes acquires considerable thickness, and 

 is then found to be composed of spongiform parenchyma, with large air- 

 cavities, the whole bounded externally by a few layers of tabular paren- 

 chyma with a corky outer surface. 



A certain number of forms occur aberrant from the type above described. 

 In Aloe the fibro-vascular bundles are so arranged as to form a kind of 

 cylinder, separating a central from a cortical parenchyma. In the Snii- 

 laceae, Dioscoreaceae, and some other Orders, the rhizomes imitate still 

 more the Dicotyledonous arrangement ; for not only do the bundles stand 

 in circles, they do not pass wholly off into the leaves, but run continuously 

 through the structure. Still there is no periodical resumption of activity 



