194 XV. THE LEAF VASCULAR BUNDLE ENDINGS. 



stomata. The two or three layers of palisade-like, elongated, 

 green cells are developed to much the same extent all round the 

 stalk, but are looser on the under side. To these follow rounded 

 cells, the outer green, the inner colourless, and becoming larger 

 inwards. In this inner cylinder of colourless cells run the 

 vascular bundles, the strongest in the vertical median plane, 

 and nearer to the under side ; the others on either side of the 

 large one, progressively diminishing in size, and each with its 

 xylem portion turned towards the centre of the leaf -stalk. The 

 larger of these vascular bundles are provided on- their outer side 

 with strings of sclerenchyma fibres. The activity of the cambium 

 has also apparently lasted longer in these bundles, and it has 

 formed inwardly secondary wood, and outwardly secondary, thin- 

 walled bast. 



It is not yet practicable for the student to demonstrate the 

 correlation of the vascular bundles of the phanerogamic leaf with 

 those of the stem ; it is sufficient here to note that the former 

 are articulated with the latter in divers ways ; that in passing 

 out of the central cylinder or stele of the stem each foliar bundle 

 is - accompanied by some of the stelar ground-tissue, and may 

 therefore be considered as a partial stele or schizostele. The 

 foliar bundle may therefore be looked upon as accompanied by 

 a sheath of ground-tissue (conjunctive parenchyma), which de- 

 creases, even as the bundle diminishes in size, with successive 

 branching. Our descriptions have shown, however, that so long 

 as it persists it is quite distinguishable from the mesophyll of the 

 leaf ; so that the leaf consists of epidermis, mesophyll and resi- 

 dues of the partial steles. In the ultimate veinlets the mesophyll 

 itself forms the sheath of the partial stele. Traced down through 

 the leaf-stalk the mesophyll joins on to the inner green cortex 

 of the stem. In leaf, as in stem, therefore, the parenchyma may 

 be considered as having two values, stelar (or conjunctive) and 

 extra-stelar (or cortical). 



Substitute Leaves. If Euta graveolens is not accessible, the 

 leaves of the Christmas Rose (Helleborus niger), or of the Lilac 

 (Syringa vulgar-is) can be recommended for study. -The Helle- 

 borus leaves are available throughout the year, and are easy to 

 cut. They show on the upper side a fairly typical palisade 

 parenchyma of broad cells, and below it a loose spongy tissue. 

 Syringa vulgar is shows a similar structure, but the leaves are 

 more difficult to cut. Several narrow strips can be cut from a 



