The Science of Plant Growing 



37 



portion is younger and more fibrous. Collectively the various members of 

 the rind are known as bark. Some of the fibres are thickened, and known 

 as hard bast (No. 5), while the inner cells remain thin-walled, constituting 

 the soft bast (No. 6). Archangel mats are made from the hard bast of the 

 younger portion of the bark of the Lime tree. Sieve tubes (No. 7) are 

 included in the hard bast, which serves to give toughness and pliability 

 to stems and branches. The bark, collectively, also serves to protect the 

 cambium from injury and the wood from decay; hence one good reason 

 for careful pruning and judicious lopping of all trees whatever. 



3*5678 9 10 11 12 



Fig. 20. Portion cut from a Branch of a Leafy Tree x about 200 (diagrammatic) 



13 



1, Superficial coat (Epidermis). 2, Cork (Periderm). 3, Cortical parenchyma. 4, Vascular bundle 

 sheath. 5, Hard bast. 6, Soft bast. 7, Sieve tubes. 8, Cambium. 9, Pitted vessel. 10, Wood 

 parenchyma. 11, Scalariform vessels. 12, Medullary sheath. 13, Medulla or pith. 



The Cambium. Even in winter, when the trees are leafless and com- 

 paratively at rest, the thin-walled cells of the cambium are small and filled 

 with protoplasm; it really constitutes the only live portion of the tree at 

 this period. It forms a thin, cylindrical jacket to the trunk of the tree, and 

 gradually tapering cylinders to each branch and twig, till continuous with 

 the small core in each live bud on the tree. The cambium descends to the 

 roots in like manner. When the temperature rises in spring the cells are 

 excited into rapid growth, and, with abundant supplies of stored food 

 close to hand, they soon reach full size, divide, grow, and multiply rapidly. 

 The cells on the inner side develop new fibre-vascular bundles, side by side, 

 in a continuous ring all round last year's wood. Those on the outside of 

 the cambium form new hard and soft bast. Thus the wood increases in 



