Bird. — Lianes of the Ancient Forest of Canterbury Plains. 325 



(ii.) Leaf-anatomy 



(a.) Shade Leaf. — Epidermis : Cells slightly elongated in axis parallel 

 to the leaf-surface ; thick cuticle ; lateral walls thinner and irregular. 

 Hypoderm : A single layer of collenchymatous cells on both leaf-surfaces 

 in the regions of the vascular bundles ; cells are large and isodiametrical. 

 Chlorenchyma : Palisade — In regions between two adjacent hypoderm 

 layers the tissue touches the epidermis and is 3 layers in thickness ; be- 

 neath the hypoderm it is composed of 2 layers. Cells are long and closely 

 packed. Tissiie occupies nearly one-half of leaf-thickness. Spongy — 5-6 

 layers thick ; cells are elongated and abutting by their ends, forming large 

 intercellular spaces ; chloroplasts are comparatively numerous. Vascular 

 bundles surrounded by thick- walled parenchymatous sheath. Mechanical 

 tissue : Each bundle-sheath is connected with the hypoderm by collen- 

 chyma, the whole forming an I-shaped girder, which is further strengthened 

 by stereome at the base of each bundle ; each half of a leaf-blade contains 

 about 5 of these girders ; at the base of the midrib the mechanical tissue 

 is very abundant. Leaf-margin is strengthened by thickening of the walls 

 of the epidermis and hypoderm ; chlorenchyma cells are elongated and 

 radially arranged. Calcium oxalate in aggregate crystals. 



(&.) Sun Leaf. — Epidermis : Cuticle very thick. Hypodermis forms an 

 uninterrupted ring beneath the epidermis. Chlorenchyma : Palisade — Cells 

 more elongated, and more closely packed. Spongy — Cells more spherical, 

 and intercellular spaces thus lessened. 



The Leaf of the Species of Rubus as a Climbing-organ. 



The value of the leaf as a climbing-organ arises from four characteristics : 

 (a.) The leaves extend from the shoot at right angles, and are therefore 

 in the position most suitable to overcome vertical strains, so that the shoot 

 may maintain its upright position and thus grow to a greater height. This 

 is shown by the great height of many adventitious shoots. In many cases 

 in which the leaves can find a support, and so prevent the shoot from slipping 

 down, shoots as slender as 1-7 cm. in diameter may grow upright for 7 m. 

 (6.) Lateral strains are overcome by means of the prickles, which, on account 

 of their hooked shape, form, efficient anchors. The effect of the strain 

 is minimized by the position of the pricldes — a position which causes the 

 strain to be distributed along the whole petiole. It is evident that the 

 prickles also are an important factor in preventing the slipping-down of an 

 upright shoot, (c.) Assisting in overcoming such strains is the downward 

 curvature of the apex of the lamina. The power of resistance of this is 

 perhaps very small in comparison v/ith that of a priclde, but it has the 

 advantage that it acts where there is little resistance by prickles, the 

 prickles being more commonly absent from the lamina, (d.) The strains 

 may also be slightly overcome by the top of a leaf coming into contact 

 with a support. This is due to the hooked arrangement which results 

 from the irregular growth of the petioles at the base of each lamina. 



The Ruhus leaves of most importance in climbing are those of R. cissoides 

 var. panperattis. These are practically devoid of laminae, and the mid- 

 ribs, armed with sharp recurved prickles, grow to a great length — oJEten 

 24 cm. These usually project from the petiole at right angles, or may be 

 variously curved and twisted, and so are highly efficient in grasping sup- 

 ports (figs. 3 and 4). In a few cases studied this tendency of the midribs 



