iJO 



LECTURE X. 



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

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

 ■within 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 time 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 Coniferse 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 thin cork membrane, which, like the epidermis, completely 

 envelopes the whole organ as a smooth layer. 



Fig. 173. — Part of transverse section of a stem of Dracana {re- 

 Jlexat) about 13 mm. tliick and i ni. high, taken about 20 cm. beneath 

 the apex. <■ epidermis; k cork (periderm); r cortical fundamental 

 tissue ; b transverse section, of a fibro-vascular bundle which bends 

 out into a leaf; m the primary fundamental tissue (pith); g the 

 primary bundles: x the zone of meristem, in which are young 

 vascular bundles — older ones g have already, partly or wholly, 

 emerged from the zone, its inner portion being transformed into 

 radially arranged fundamental tissue {si). 



