LEAF OF PINUS 



343 



spongy tissue (s.), is traversed by the single bundle (b.), which 

 exhibits the usual dorsiventral structure. On either side of the 

 xylem, however, are occasional spirally or reticulately thickened 

 elements (i.), which sometimes bear bordered pits. These dead 

 cells constitute what is called transfusion tissue, and may serve 

 both for the lateral conduction of water, thus compensating for 

 the absence of side- veins, and as water-reservoirs. 



The epidermal cells of the P«ms-leaf (Fig. 199, A-D) also 

 have a very pronounced cuticle {Cu.), and, in this case, are so 

 strongly thickened that their cavity is often reduced to a mere 

 dot (ep.) ; beneath the epidermis is a strongly thickened hypo- 

 derm (A.). The deeply sunken stomata (St.) occur at intervals 

 all round the leaf, as seen in transverse section, but, if the leaf 

 be examined whole with a lens, they are seen to be arranged in 

 longitudinal rows. The mesophyll contains several prominent 

 resin-canals (r.) and consists of more or less isodiametric cells, 

 characterised by the presence of inwardly projecting folds of 

 the wall (»'.), which increase the surface for absorption of carbon 

 dioxide (cf. p. no). It will be noticed that, apart from the 

 respiratory cavities of the stomata, there are few intercellular 

 spaces between the cells of the mesophyU. This tissue, how- 

 ever, consists of successive transverse lamellae, and these are 

 separated from one another by narrow air-spaces (Fig. 198, E). 

 The twin-bundles (in some species of Finns, as well as in 

 Picea and Larix, there is only a single one) are connected 

 by a group of thick-walled mechanical cells [Scl.) and are 

 embedded in an extensive mass of tissue bounded by a well- 

 marked bundle-sheath (5.). This tissue consists, in large part, 

 of ordinary living parenchymatous cells containing starch- 

 grains. Scattered among these are the dead and empty cells 

 of the transfusion tissue (cf. especially Fig. 199, C, tr.), in this 

 case bearing small bordered pits (&.), which are seen both in 

 section and from the surface. At the outer edge of the phloem 

 of each bundle is a group of cells, whose dark contents are 

 rich in proteins (a.), and which probably assist in the con- 

 duction and storage of elaborated food-material. 



^It may be pointed out that, whilst many of the features of 

 Coniferous leaves are those usually found in plants with reduced 

 transpiration, others (such as the limited vascular supply and 



