75 



The Tegumentary System comprises the epidermis, or entire Tegu 



i -c J.-L i j. v i, n -_t -i System. 



outer skin 01 the plant, which usually consists ot a single " 



continuous layer of cells. These fit close together and, in the 



serial parts of plants at all events, have their external walls 



more or less thickened and cuticularised, i.e. converted into 



cutin, a substance resembling cork but which is more resistant 



to the action of caustic potash and strong sulphuric acid. 



When such cuticularised external walls are very thick their 



outermost layers are more cuticularised than the rest and 



form the so-called cuticle, which can often be detached as a 



separate membrane from the other thickening layers. The 



epidermis is also frequently provided with an additional external 



covering in the shape of deposits of wax, or various forms of 



hairs, the latter being outgrowths of the epidermal cells. The 



cells of the epidermis do not usually contain chlorophyll. It vascular 



was pointed out, in Part I above, that the firm nerves, to Bundle 



which the venation of the leaves is due, pass into the stem, ys 



where they join on to stouter similar strands which are again 



continued into the root, and also that these strands were 



called vascular bundles. In some Balsams, the stems of which 



are more or less transparent, these strands passing down the 



stem can be seen with the naked eye and in other cases they 



may often be isolated from young stems which have been 



allowed to rot in water by carefully washing away the softer 



tissue. Vascular bundle skeletons of leaves, also, are often 



naturally produced as the leaves slowly decay. These vascular 



bundles together constitute the Vascular Bundle Tissue System. 



A.11 tissue which does not belong either to the Tegumentary Fundamental 



or Vascular Bundle System is classed as Fundamental, and system. 



consists usually of parenchyma, but also often contains collen- 



chyma and sclerenchyma. 



70. A vascular bundle consists structure o 



of two portions, the Xylem, or Wood portion and the Phloem, Bundles* 

 or Bast portion. The former consists chiefly of vessels and 

 tracheids, or sometimes of tracheids alone, with some paren- 

 chyma, while the phloem portion contains sieve-tubes and 

 parenchyma. Fibres may also be present in bothxylem and 

 phloem. Most commonly the xylem and phloem are in contact 

 on one side only and the bundle is said to be collateral. In 

 the stem the xylem and phloem portions of a collateral bundle 

 are found on the same radius of the stem, the xylem being 

 nearest the centre of the stem and the .phloem nearest the 

 circumference. In some cases the xylem has phloem not only 

 on the outside, but also on the inside, the xylem being placed 



