94 BOTANY PART I 



the peripheral strands has itself the form of an I-girder (Fig. Ill, 3) ; only the outer 

 bars of this consist of mechanical tissue, the connection being usually made by a 

 vascular bundle (system of compound I-girders). These arrangements are on the 

 whole better shown in the stems of Monocotyledons than in the primary structure 

 of the stems of Dicotyledons and Gymnosperms. In the latter the rigidity can be 

 increased by the secondary thickening. In stems which are green and carry on 

 assimilation the mechanical tissue is somewhat removed from the epidermis, being 

 separated from the surface by the green tissue for the functions of which light is 

 necessary ; in other cases the mechanical and assimilatory tissues share between 

 them the peripheral position (Fig. Ill, 2). 



The innermost layer of cells of the cortex in the mature subaerial 

 stems of land plants is not usually specially characterised. There is 



FIG. 111. Rigidity against bending. 1. Transverse section of a young twig of Sambucus ; c, collen- 

 chyma. 2. Part of the transverse section of a haulm of grass (Molinia coerulea); Sc, ribs of 

 sclerenchyma ; Sc R, sclerenchymatous ring connecting them laterally ; A, green assimilatory 

 tissue ; MH, pith-cavity. 3. Diagram of double girder on a larger scale, g, g, Girders ; 

 /, connecting tissue represented by the vascular bundle. (1 and 2 after NOLL.) 



then no sharp limit between cortex and central cylinder. This layer 

 may, however, be developed as a starch sheath, as a typical 

 endodermis (especially in the subterranean shbots of land plants and 

 in the stems of aquatic plants), or as a cutis. If developed as a 

 STARCH SHEATH (s/, Fig. 112 A, B) its cells contain large, easily- 

 movable starch grains. 



The starch sheath is often present in the young shoots only and disappears or 

 remains limited to certain spots in the older condition. In place of a common 

 starch sheath or endodermis such sheaths may be found around the separate 

 bundles (Fig. 119 pp), or there may be single rows of cells containing easily- 

 movable starch. 



Central Cylinder. This is composed of various tissues. The 

 parenchyma, in accordance with its deep-seated position, is almost 

 or quite colourless, and serves mainly for conduction or storage. 

 Sclerenchyma frequently is present. The most important parts of 

 the central cylinder are, however, the VASCULAR BUNDLES which serve 

 for the carriage of water with the necessary salts from the roots to 



