52 



Elementary Botany 



enter the stem from the bases of the leaves being continuous 

 with the bundles present in them. At first they are narrow. 

 They grow inwards, and then pass down the interior of the 

 stem, gradually increasing in diameter. At length, having 

 attained their largest size, they begin to curve outward again, 

 thinning as they do till they end at the exterior (fig. 73). 



/! 



Fig. 73. — Course of the 

 vascular bundles of Iris 

 in longitudinal section 

 (diagrammatic). 



Fig. 74. — Transverse section of fibro-vascular bundle of 

 Zea, Mais : /, thin-walled parenchyma of ground tissue 

 of, a, outer and, i, inner part of stem, with thick-walled 

 prosenchymatous ground tissue internal to it ; g^ g, large 

 pitted vessels ; s, spiral vessel ; r, isolated ring of annular 

 vessel ; /, air cavity ; p, z/, cambiform tissue or soft bast. 

 (After Sachs.) 



The bundles are closed, containing no cambium, hence 

 after their formation they cannot increase in size. A section 

 across the stem will show the younger bundles within and the 

 older ones without. Such a mode of growth is often termed 

 endogenous, but it must be borne in mind that it is only at a part 

 of their passage that this is true, as at the commencement and 

 end they grow outwards. 



In annual and herbaceous monocotyledons the ground tissue 



