334 



PRIMARy ARRANGEMENT OF TISSUES. 



Fig. 156. Cross-section through the vascular bundle In the midrib of the leaf of Olea 

 Europaea (375). s-s the phloem, consisting of wide (parenchymatous ?) cells, and scattered 

 groups of very narrow elements (sieve- tubes ?) ; comp. p. 3 2 5 . f-f sclerenchy matous fibres, 

 forming a girdle round the outer edge of the phloem, and occurring scattered on the 

 inside of the xylem. The very dense xylem borders on the phloem internally ; the 

 primitive elements at its inner edge do not appear clearly; its. larger external portion 

 consists of radial rows of thick-walled pitted tracheae, which alternate with bands of 

 parenchyma. The latter are indicated by the granular dotting of the lumen, p paren- 

 chyma. 



Fig. 156. 



Fig. 157, Cross-section through a vascular bundle in the leaf of Welwitschia mirabilis 

 (145). An uninterrupted zone of very thick-walled and elongated sclerenchymatous fibres 

 surrounds at / the outer edge of the phloem, and at /' the inner edge of the xylem. 

 Inside the zone/ follows the phloem, which is crescent-shaped in cross-section, consisting 

 of narrow, radially arranged, elongated elements ; their structure could not be detected 

 with certainty, nor are they drawn quite accurately in the figure, because the great 

 swelling of the membranes makes it impossible to spread out the cross-section of the 

 phloem, in one plane, in such a preparation as that figured. The inner portion of the 

 xylem enclosed by the fibrous sheath/' consists of tolerably wide, elongated prismatic 

 cells, connected without intercellular spaces, with thick almost gelatinously soft mem- 

 branes. Between them are inserted numerous, very narrow, compressed and distorted, 

 spiral and annular tracheides, with thick, distorted, fibrous thickenings ; jj> ; they doubt- 

 less represent the primitive elements of the bundle. Further outside follow the persistent 

 tracheae, placed in tolerably regular rows, here and there alternating with delicate cells, 

 and in general increasing in width in each row from within outwards ; first annular and 

 spiral tracheae, with dense and very broad thickening layere, then reticulated and (at g) 

 large pitted vessels, with bordered pits and round perforations in the cross-walls, t, i 

 are the rows of pitted and reticulated tracheae in cross-section ; they surround the bundle 



