Lecture XXII. 



181 



at the base of each season's growth at the end of the lateral 

 branches and thus provide for the dorsiventral branching of the 

 lateral branches. 



The stem is prolonged downwards into the soil by a large root. 

 The downward extension of this tap-root depends on the depth 

 of the soil. It sends off side roots and these branch repeatedly. 

 The finest branches of the roots are delicate and thread-like. 



The green, or foliage, leaves are long and narrow. Each is 

 a half cylinder in form and tapers towards both ends. Its flat 

 surface at the base is turned towards its neighbour in the tuft, 

 its upper part is loosely twisted. A single vein runs down the 

 middle of each leaf containing two small vascular bundles, or 



be A 9 \ c , j d e a 9 _ 



b 



FIG. 52. Pinus silvestris, leaf transverse section, x 58. a, cuticle ; b, epi- 

 dermis ; c, stomata ; d, mesophyll-cells with infolded walls ; e, resin- 

 ducts ; /, air-spaces ; g, endodermis ; h, transfusion tissue ; i, wood or 

 xylem ; &, bast or phloem. 



conducting tracts. Outside the vein is the mesophyll of the 

 leaf, and it is protected by a fibrous layer, which is in turn coated 

 by the epidermis. The epidermis is composed of thick- walled cells 

 forming strong fibres elongated in the direction of the length of the 

 leaf. The outer walls of its cells are covered with a firm cuticle. 

 The epidermis is perforated by longitudinal rows of stomata, 

 which lead into air-cavities in the mesophyll. The stomata are 

 bordered, as in the case of the Archegoniates, by two guard-cells. 

 These are sunk below the general level of the epidermis, their 

 walls are much thinner than those of the other epidermal cells 

 and they contain numerous chloroplasts, while the other cells 

 have none. The mesophyll-cells are peculiar, they are isodia- 

 metrical and fit fairly closely together. From their thin walls 



