90 BOTANY 



and all meristematic tissue which can be shown to have been developed 

 directly from such promeristem is termed PRIMARY. A primary 

 meristem, in the midst of a completely developed tissue, may still retain 

 its meristematic character. Fully differentiated tissue is designated 

 permanent tissue in contrast to meristematic tissue. At times, per- 

 manent tissue may again become capable of division, and in that con- 

 dition is called secondary meristem. 



Tissue Systems 



A mass of tissue so united in the body of a plant as to form a 

 distinct histological unit constitutes a tissue system. In the more 

 highly organised plants three such systems may be distinguished — the 



TEGUMENTARY SYSTEM, the VASCULAR BUNDLE SYSTEM, and the 

 FUNDAMENTAL TISSUE SYSTEM. 



The tissues which make up the different tissue systems are dis- 

 tinguished as PRIMARY and SECONDARY, according as they are derived 

 from the promeristem or secondary meristem. 



The primary tissues of the tissue systems will be considered first. 



The Primary Tissues 



The Tegumentary System. — InthePteridophytes and Phanerogams 

 the plant body is covered by a distinct outer tegument or epidermis. 

 On the inside, the epidermis, which is usually composed of but a single 

 layer of cells (Fig. 87, e), is sharply marked off from the adjoining 

 tissue, while on the outside it is much thickened. This is especially 

 the case in all aerial parts of plants adapted for a long life, but on the 

 more perishable parts of a plant, such as the floral leaves, or on those 

 parts more protected, as the root, the cells of the epidermal layer are 

 generally thin -walled or only slightly thickened. Even when the 

 external walls of the epidermal cells are considerably thickened, the 

 side walls, at least in part, remain unthickened. The external walls 

 are also more or less cuticularised, while their outermost layer, which 

 is more decidedly cuticularised and capable of withstanding even the 

 action of concentrated sulphuric acid, extends as a CUTICLE continuously 

 over the surface of the epidermis. The cuticle has its origin in the 

 primary walls of the younger epidermal cells, which, during the increase 

 in size of the plant, become very much distended and at the same 

 time strengthened by the deposition of cutin. The cuticle frequently 

 becomes folded, and so assumes a striped appearance (Fig. 107). 

 Plants in dry climates, or so situated that, for any reason, transpiration 

 from their outer surfaces must be diminished, are characterised by 

 the extraordinarily thickened and cuticularised walls of their epi- 

 dermal cells. In some of the Grwmineae, Equisetaceae, and many other 



