112 VASCULAR SYSTEM OF LEAF 



usually colourless with a homogeneous mesophyll, and often 

 possess no veins. 



Movement of chloroplasts in conformity with the intensity of 

 the illumination is well seen in a few plants (e.g. Duckweed, 

 Lemna; Moss-leaves, etc.). In these cases the chloroplasts 

 occupy a profile position on the vertical walls when the light is 

 intense, whilst when weak the chloroplasts pass to the horizontal 

 walls, so that their full surface is presented towards the source of 

 illumination. Similarly the leaves of some plants (e.g. Bracken ; 

 Yorkshire Fog, Holcus mollis) take up fixed light-positions, at 

 an oblique angle to strong sunlight, so that much of this is 

 reflected from the leaf-surface and the chloroplasts are con- 

 sequently protected. When growing in shady situations, 

 however, the leaf-blades are placed at right-angles to the in- 

 cident rays. 



The vascular tissue of the leaf is very extensive, forming 

 an irregular network (reticulate) in Dicotyledons and a very 

 regular (parallel) system in most Monocotyledons. 1 The repeated 

 branching facilitates not only the delivery of water and mineral 

 salts to all parts of the leaf, but also the rapid removal of elabor- 

 ated food-substances. The vascular system, however, also con- 

 stitutes a supporting skeleton for the lamina, in which it is often 

 aided by accompanying mechanical tissues ; the latter are found 

 especially in the larger veins, and consist of strands of collenchyma 

 (Fig. 51, M.) or sclerenchyma, which run both above and below 

 the vascular bundles or sometimes on the lower side only. 



A transverse section through one of the principal veins of 

 the Fuchsia-leai shows a single collateral bundle (cf. p. 76) 

 enveloped in a sheath of one or more layers of large transparent 

 thin-walled parenchyma-cells (Fig. 51, Sh. ; cf. also Fig. 82, V.) ; 

 the accompanying collenchyma (M.) is developed especially on 

 the lower side. The xylem, which is adjacent to the palisade 

 tissue, consists of rows of vessels alternating with wood-paren- 

 chyma, the protoxylem being directed towards the upper epi- 

 dermis ; the phloem is of the normal type and lies towards the 

 lower side. Between xylem and phloem a cambium can often 

 be recognised (Fig. 51) ; this is especially the case in evergreen 



1 The Cuckoo-pint (Arum maculatum] and Black Bryony (Tamus com' 

 munis], for instance, have a venation similar- to that of Dicotyledons, 



