128 ANATOMY OF STEM, ROOT AND LEAF 



them. Tyloses or peculiar bladder-like protrusions from the 

 adjoining thin-walled cells also block up the cavities of the 

 vessels. Tannin and colouring matters are also present in 

 the cell-membranes and cavities of the heart-wood of many trees. 

 Some of these substances act as preservatives against the attacks 

 of insects and fungi, and to them the durability of the heart- 

 wood is due. Whilst in oak, ash, elm, walnut, apple, laburnum, 

 larch, various pines, and many other trees a considerable 

 difference in colour is observable between the heart-wood and 

 splint-wood ; in beech, hornbeam, sycamore, lime, silver-fir, and 

 spruce no such distinction of colour is visible to the naked eye ; 

 but the heart-wood of these trees can frequently be distinguished 

 from the splint-wood by its dryness, although small numbers of 

 living cells are sometimes present in wood of this character right 

 through to the pith even in trees of considerable age. Trees of 

 the latter type are more liable to become hollow than those in 

 which a coloured heart-wood is present. 



553^,,____,,^-^S2g35^^6 («) Periderm.— In annual 



and perennial herbaceous 

 stems, the epidermis and 

 primary cortex grow at the 

 same time as the cambium 

 is increasing the bulk of wood 

 and bast in the vascular 

 cylinder, so that a continuous 

 covering is maintained in 

 such stems in spite of the 

 internal growth in thickness. 

 Even in some woody stems, 

 such as mistletoe and holly, 

 the epidermis persists and 

 keeps pace for years with the 

 growth of the wood and bast within. In the majority of woody 

 stems, however, the epidermis and primary cortex are ruptured 



Fig. sg. — Transverse section through peri- 

 derm of a young black currant shoot, a Phel- 

 logen ; c cork ; b phellod^rm just forming ; f 

 bast of the stem ; ^withered primary cortex ; 

 epidermis. (Enlarged 270 diameters.) 



