VASCULAR SYSTEM OF LEAF 



11^, 



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. i8. A, 

 p. 37), the amount 

 of wood-paren- 

 chjnna diminishing 

 till it dies out. 

 Thus the bundle- 

 ends usually consist 

 only of tracheids 

 surrounded by the 

 sin gl e -1 a y ered 

 parenchyma- 

 sheath. 



In larger leaves 

 [e.g. Sunflower) the 

 midrib often con- 

 tains several 

 bundles, and ex- 

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

 {Rheum rhaponticiim) , where quite a large number of strands 

 occur. On the whole the vascular supply is proportional to 

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

 In small leaves [e.g. Mouse-ear Chickweed, Cerastiimi, Fig. 55) 

 the 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 

 8 



Fig. 54. — Diagrams showing the petiolar struc- 

 ture of A, Sea Holly [Eryngiiwi maritinium) ; 



B, Dog's Mercury {Mercurialis perennis) ; 



C, Black Bindweed (Polygonum convolvulus) ; 



D, Bishop's Weed {/Egopodium podagraria). 

 The shaded part of the bundles represents 

 xylem, the unshaded part phloem. Sc, 

 sclerenchj'ma. 



