MINUTE STEUCTtJRE. 



61 



111 the fundamental tissue, if the cells are arranged more or less 

 distinctly in rows, and bounded by flat or curved Avails, and are not 

 much longer than broad, the tissue is called parencliyma or paren- 

 chymatous tissue ; of such is the soft tissue of the leaves, etc. But if the 

 cells are pointed at their ends, much longer than broad and dovetailing 

 into one another, it is called proseneJiymatous ; of such are the ligneous 

 cells of the stems and rhizomes of all inonopodial orchids, also of 

 Cattleya, Lselia, Epidendrum, and many others. The differentiated tissues 

 of the vascular bundles may be ranged under two groups called the 

 bast and wood portion (technically the "phloem" and "xylem"); 

 they are separated by caiiihium when there is any. The bast consists 

 generally of thin-walled cells and tubes for the conveyance of nutriment; 

 the xylem or wood portion has generally thickened cell-walls which 

 become hard and ligneous for the purpose of support. The cambium 

 consists of cells capable uf further development; it has the power of 

 forming new cells, and of furnishing the material for the production 

 of new permanent tissue eitlier of bast or of wood in addition to that 

 already in existence. 



The general structure of the leaves of orchids may be thus sketched. 

 There is a central spongy mass, the " mesophyll," consisting of cells 

 not always arranged in the same way nor of the same size and 

 form, but generally in easily recognisable layers. A few are empty or 

 filled with air ; some are filled with water ; some contain food in the 

 shape of starch grains ; some are provided with green colouring matter, 

 or chlorophyll as it is called, to which leaves owe their colour, and in 

 no small degree their vitality ; some are charged with red or purple- 

 coloured fluid ; some give shelter to crystals of various forms. This 

 central mass is traversed longitudinally by the fibro-vascular bundles 

 or veins, including sundry thinner and more delicate vessels of various 

 modifications which play a part in the transmission of nutritive juices from 

 one part to another. The whole is protected on each side by a skin 

 or epidermis which, besides being a protective organ, allows of the 

 passage in and out of the leaf of air and vapour by means of minute 

 pores or apertures (the stouiata) which are generally most numerous on 

 the under surface and which open or close according to the dryness or 

 moisture of the atmosphere.* 



The illustrations of minute structures of the leaves, stems and roots 

 of some well-known orchids here given should now be intelligible to 

 the general reader, and some obvious relations between the structure 

 and functions may be pointed out. 



Leaves. — In every figure the same numerals indicate the same tissue 

 or structure, viz., 1, the upper, 2 the lower epidermis; 3, the 

 parenchyma or cellular tissue of the leaf, sometimes called the 



* Card. Chron. XXI II. (1885), p. 607. 



