68 GENERAL REVIEW OF THE OKCHIDE.E, 



for palisade structure, l>ut tliey contain no cliluro[)hyll, and are colourless 

 and filled with water. lieneath them is the chl(n'ophyll layer of ovoid 

 ox globular closely packed cells, in the midst of wliich are the fibro- 

 vascular bundles ; the cells beneath these contain a purplish colouring- 

 fluid. Fig. 11, C. insigne closely resembles C. nivpum, except that the 

 cells of the mesophyll tissue arc more uniform, and the lowermost 

 contain no colouring fluid. Shade and a copious supply of water are 

 the cultural indications to be derived from this structure. 



Stems. — Transverse sections of the stems of orchids show generally the 

 normal monocotyledonous structure. There is a well-marked epidermis 

 consisting in stems of more than one year's duration of closely packed 

 cells, covered by a hard cuticle more or less thickened ; underlying the 

 epidermis is a l)elt of corky tissue enclosing the central mass of 

 parenchyma, tlie cells of wliich are soft and thin-walled ; and in the 

 last-named tissue arc imbedded the fibro-vascular bundles more or less 



Fig. 12. A frafiinent ot transverse section of stem of Vetidrohium nobile, enlarged 30 diameters. 

 1, epidermis ; 2, flbro-vascular bundles ; 3, fundamental tissue. 



isolated as shown in diagram of transverse section of stem of Lcelia pur- 

 purata (Fig. 13 A). In addition to these there is in some cases a layer 

 of corky tissue developed between the epidermis and the fundamental 

 tissue as shown in Figs. 14 and 15. Like the fibro-vascular bundles 

 of the leaves those of the stem always consist of at least two elements, 

 one of thin-walled cells called the bast or phloem, and the other, the 

 xylem, of more or less elongated, thick-walled (prosenchymatous) cells 

 that become hard and ligneous. The bundles of the stems are much 

 more closely packed and usually of larger size than those of the leaves, 

 and fill up a great part of the central mass that is surrounded by the 

 corky tissue and impait to old stems their hard and ligneous texture. 

 In the pseudo-bulbous and swollen stems of many orchids occur many 

 cells rich in mucilage and others which are especially adapted for the 

 retention of Avater and as storage for reserve material to be utilised 

 by new growths. 



