170 



LECTURE X. 



the roots. Now the young, thin stem is traversed, as in ^lonocotyledons generally, 

 by isolated closed vascular bundles, which, originating near the surface, ascend 

 M'ithin the stem in a radially oblique direction, reach about to the centre, and then 

 curve out somewhat suddenly into the leaves (cp. Fig. 130). The paths of the 

 numerous bundles arising at various heights and curving in and out, thus cross : 

 and it is scarcely conceivable how, under these circumstances, a cambium 



layer, connected as in the true wood 

 plants, and at the same lime running 

 through the vascular bundles, could 

 come into existence. As a matter of 

 fact, the growth in thickness of these 

 plants is initiated and carried forward 

 in a somewhat different manner. A 

 zone of the fundamental tissue, annular 

 in transverse section, outside of which 

 there lies a thin cortical tissue, becomes 

 transformed by radial growth and the 

 appearance of tangential partition-walls 

 into a tissue capable of division — a 

 meristem; the activity of which cer- 

 tainly possesses great similarity to that of 

 the cambium. Some considerable differ- 

 ences, however, are apparent. Apart 

 from the fact that the development of 

 secondary cortex from this meristem 

 is generally very inconsiderable, no mass 

 of wood, homogeneous and compact 

 as in the Coniferae and Dicotyledons, is 

 deposited on the inner side of the 

 continually widening meristem circle ; 

 but the products of the layer of meri- 

 stem are secondary vascular bundles, 

 consisting of sieve-tubes and tra- 

 chei'des. These secondary vascular 

 bundles exhibit a sinuous undulating 

 course, anastomose radially and tangentially, and thus form a dense network, 

 the meshes of which are filled with radially elongated parenchyma which originates 

 from the zone of meristem, and, as is easily observed, corresponds to the medullary 

 rays of true wood. With this growth in thickness is also connected a develop- 

 ment of periderm at the surface of the organ; this, however, is usually confined 

 to the production of a tliin cork membrane, which, like the epidermis, completely 

 envelopes tlie whole organ as a smooth layer. 



Fig. 175.— Part of trnnsverse section of a stem of Dracann {>■ 

 ßexa^) about i^mm. thick and i ni. liigli, taken about 20 cm. beneat 

 the apex, f epidermis; /■ cork (periderm); ?■ cortical fundament- 

 tissue; * transverse section of a fibro-vascular bnndl»' which I'.-nr 

 out into a leaf; m the primary fundamental ti^-'i (j ul:i ; ; il 

 primary bundles: x the zone of meristem, in \\' ; h it. vmhi 

 vascular bundles— older ones 4" have already. | .iii.\ . r i\li II 

 emerged from the zone, its inner portion being transformed in 

 radially arranged fundamental tissue {st). 



