36 VEGETABLE ORGANS. 



These structures are of different composition, as 

 may be best seen in longitudinal sections. The pith 

 and the medullary rays consist of cellular tissue, the 

 cells being mostly rounded in the former, and more 

 closely pressed together and squarish in the latter. 

 The medullary sheath consists of vascular tissue ; and 

 the wood, of wood- cells traversed longitudinally by 

 bundles of vascular tissue and ducts, the latter being 

 larger and more distinct towards its outer boundary. 

 The liber is composed of woody fibre, and the outer 

 bark of cellular tissue. 



The new woody matter being deposited outside the 

 old, between the bark and the previously formed 

 layer, gives origin to the term exogen (efo), outside, 

 yevvaw, to produce). These structures may be ex- 

 amined in the section of a branch of the lime-tree 

 or lilac. 



In the Monocotyledons or Endogens (PL I. fig. 37), 

 there is no distinct bark, nor pith, nor medullary 

 rays the entire stem consisting of cellular tissue 

 with isolated bundles of fibro-vascular tissue scat- 

 tered through it. Moreover the new substance is 

 added to the centre of the stem, or within the old ; 

 hence the term endogen (eVcW, within, ryevvdw) . A 

 section of a piece of cane will exhibit this structure. 



To examine the structure of stems, sections must 

 be made in various directions. The relative position 

 of the component parts of a stem are best seen in a 

 transverse section ; but the structure of the tissues is 

 most evident in longitudinal sections, and under the 

 higher powers. The annual rings of the Exogens are 

 best observed in transversely sawn-off pieces of per- 

 fectly dry stems, which have been polished with sand- 

 paper, and varnished with spirit varnish. 



Roots. The structure of roots is very similar to 

 that of stems ; there is, however, no distinct pith, nor 

 are there stomata on the epidermis ; and the vessels 

 are replaced by ducts. The very fine rootlets or 



