ANATOMICAL STRUCTURE OF THE LEAVES. 



33- 



Fig. 15, Transverse section of leaf of Picea excelsa 



Fig. 15 represents a transverse section of a leaf of the common 

 Spruce Fir, Picea excelsa, magnified fifty diameters. The row of small 

 polygonal cells with thickened walls immediately beneath the epidermis 

 or skin is technically called the hypoderm, the individual cells, 

 strengthening cells. In the common Spruce the hypoderm consists of 



a single layer of cells, but among 

 the Pines there are instances of 

 the hypoderm consisting of two 

 and even more layers, whilst in 

 some species of Tsuga it is either 

 absent or restricted to certain 

 parts of the leaf. The shape of 

 the hypoderm cells varies con- 

 siderably iii the different genera. 

 The small shaded cells in the 

 centre of the leaf denote the 

 elements composing the single 

 fibro- vascular bundle /, passing along the mid-rib ; it is separated from 

 the principal tissue of the leaf by a well-defined circle of small cells 

 (endoderm) ; the large circular opening r, beneath the fibro-vascular 

 bundle and close to the epidermis, is the resin-canal ; this is lined 

 with a layer of small cells called epithelium cells. In sonie leaves 

 of this species, the resin-canal is either entirely absent or does not 

 traverse the whole length of the leaf. The substance of the leaf filling 

 up the greater part of the area of the transverse section is called the 

 parenchyma or mesophyll. As shown by the figure, the mesophyll 

 in Picea excelsa is wholly cellular and fairly uniform throughout ; the 

 greater part of the cells composing it contain the chlorophyll granules 

 which give the green colour to the leaf and perform the important 

 functions already alluded to. 



Fig. 16 represents a transverse section of a leaf of Tsuya Brunoniana, 

 the Himalayan Hemlock Fir, magnified thirty diameters. In this case the 

 hypoderm is confined to the two marginal sides of the leaf. In the 



fibro-vascular bundle / in 

 the centre, the principal 

 elements called phloem and 

 xylem are more clearly 

 differentiated than in the 

 Spruce Fir ; immediately 

 below it is the single 

 resin-canal r lined with a 

 layer of very minute 



epithelium cells. In the Hemlock Firs the cellular tissue forming the 

 mesophyll is not of uniform structure, the cells immediately beneath the 

 epiderm being mostly elongated in a vertical direction to it. This form of 

 cell is strongly marked in Tsut/a Brunoniana ; from their formal 

 appearance they have received the name of palisade-cells. 



Fig. 17 shows a transverse section of a leaf of the common Silver. Fir, 

 Aid?* pectinata, magnified thirty-two diameters. Here the hypoderm 

 consists of a single row of small cells continuous only in the centre of 

 the leaf above and beneath the midrib and much interrupted or 

 entirely absent in other parts. The midrib consists of two fibro-vascular 

 bundles surrounded by a sheath (endoderm), whilst the resin-canals r r 



Fig. 1(3. Transverse section of leaf of Tsuga Brmioniana. 



