VASCULAR SYSTEM OF LEAF 



leaves (e.g. Holly) which remain on the plant for more than one 

 year. 



In passing to the finer and finer ramifications of the vascular 

 system a gradual simplification in structure is apparent. The 

 differentiation of the phloem becomes less and less distinct, its 

 place being taken by a more or less uniform tissue of thin-walled 

 elongated cells, whilst at the ultimate terminations of the bundles 

 it often disappears 

 completely. Simi- 

 larly the xylem- 

 vessels gradually 

 give place to re- 

 latively short spiral 

 or reticulate tra- 

 cheids (Fig. 18, A, 

 p. 37), the amount 

 of wood-paren- 

 chyma diminishing 

 till it dies out. 1& 

 Thus the bundle- \<* 

 ends usually consist 

 only of tracheids 

 surrounded by the 

 single-layered 

 parenchyma- 

 sheath. 



In larger leaves 

 (e.g. Sunflower) the 



lidrib often con- 



ains several 

 mndles, and ex- 

 treme cases are seen in the Docks (Rumex) and the Rhubarb 

 (Rheum rhaponticum) , where quite a large number of strands 

 )ccur. On the whole the vascular supply is proportional to 

 the size of the leaf, and this is true also of that of the petiole. 

 [n small leaves (e.g. Mouse-ear Chickweed, Cerastium, Fig. 55) 



le latter frequently contains but a single vascular strand 

 orientated as in the lamina, whilst in larger ones there are 

 several bundles usually grouped in the form of a C with the 



D 



FIG. 54. Diagrams showing the petiolar struc- 

 ture of A, Sea Holly (Eryngium maritimum) ; 



B, Dog's Mercury (Mercurialis perennis) ; 



C, Black Bindweed (Polygonum convolvulus) ; 



D, Bishop's Weed (JEgopodium podagraria). 

 The shaded part of the bundles represents 

 xylem, the unshaded part phloem. Sc., 

 sclerenchyma. 



