254 



THE ORCHID WORLD. 



[October, 1916. 



the Monocotyledonous Division, that to which 

 the Orchideas belong, the fibro-vascular 

 bundles of more than one year's old stems as 

 in Vanda, Cattleya, Dendrobium, etc., and 

 also in the pseudo-bulbous species, are 

 isolated and separated from each other by 

 fundamental tissue ; and in the leaves they 

 are either parallel to each other or symmetri- 

 cally placed on each side of the mid-rib. 



In the fundamental tissue, if the cells are 

 arranged more or less distinctly in rows, and 

 bounded by fiat or curved walls, and are not 

 much longer than broad, the tissue is called 

 parenchyma or parenchymatous 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 prosenchymatous ; of such 

 are tlie ligneous cells of the stems and 

 rhizomes of all monopodial Orchids, also of 

 Cattleya, La?lia, 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 cambium 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 of 

 further development ; it has the power of 

 forming new cells, and of furnishing the 

 material for the production of new permanent 

 tissue either 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," con- 

 sisting 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 pro- 

 tected 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 stomata) 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 rela- 

 tions between the structure and functions may 

 be pointed out. 



Leaves. — In every figure the same 

 numerals indicate the same tissue or structure, 

 viz., I, the upper, 2 the lower epidermis; 3, 

 the parenchyma or cellular tissue of the leaf, 

 sometimes called the mesophyll ; 4, the fibro- 

 vascular bundle of the mid-rib ; 5, smaller 

 bundles parallel with the mid-rib (veins) ; 6 

 (where present), still smaller bundles (vein- 

 lets). The epidermis is always covered by a 

 structureless cuticle developed from the cell- 

 walls and forming an unbroken layer over the 

 whole leaf and resisting too rapid evapora- 

 tion of the fluids within. Fig. i illustrates a 



Fig. 1. Transverse section of leaf of Dendrobium nobile at 



mid-rib, enlarged 30 diameters. 

 1, upper, 2, lower epidermis; 3, parenchyma or soft tissue containing 

 chlorophyll granules ; 4. fibro-vascular bundle of the mid-rib; 5, two 

 smaller bundles. 



transverse section of a leaf of Dendrobium 

 nobile enlarged 30 diameters. Immediately 

 underlying the upper epidermis is a layer of 



* Card. Chron. XXIII. (1885). p. 607 



