256 



THE ORCHID WORLD. 



[October, 1916. 



leaf IS thick and fleshy, the epidermal cells 

 small and closely packed ; the hypoderm or 

 water cells are large ; the air cavities are also 

 large and nearly equidistant from both 

 surfaces ; the fibro-vascular bundles are 



Fipr- 4. Fragment of epidermis and underlying cells of Cattleya 

 Mossiae, magnified 200 diameters : the outer surface of the epidermal 

 cells is developed into a thick cuticular layer (a) covering the whole 

 surface and which has a peculiar striated structure in the external 

 half. This thick cuticle impedes excessive evaporation. 



sunlight, and the smaller hypoderm or water 

 cells a shorter duration of the rainy season 

 than in the caSe of Cattleya Mossiae, the other 

 circumstances remaining much the same. 



Fig. 6. Transverse section of leaf of Odontoglossum crispum at 

 mid-rib, enlarged 50 diameters. The numerals as before. 



numerous and somewhat similarly grouped on 

 each side of the principal or mid-rib-bundle. 

 This structure may be connected with the 

 occurrence of a heavy rain-fall, which we know 

 to be the case in its native home, and also 

 exposure to direct sunlight. Fig. 4 is a 

 fragment of the upper epidermis and under- 

 lying cells, magnified 200 diameters, to show 

 the structure more clearly. 



Fig. 5, L^lia purpurata. The thickened 

 cuticle and elongated cells filled with 

 chlorophyll imply a greater exposure to direct 



Fig. 5. TransTerse section of leaf of Laelia purpurata across the 



mid-rib, enlarged 40 diameters. 

 1 . upper. 2, lower epidermis, both with thickened cuticle; 3, parenchyma 

 or soft tissue (mes^phyll). containing chlorophyll granules (not shown 

 in figure) ; 4, large vascular bundle of the mid-rib ; 5, two smaller 

 vascular bundles parallel with the mid-rib ; 6, smaller vascular 

 bundles (veinlets). 



Leaves of Vandeae. — Fig. 6, Odonto- 

 glossum crispum. Here the cuticular covering 

 is thin, the capacity for resisting evaporation 

 and the storage capabilities as regards water 

 relatively small, and the fibro-vascular bundles 

 numerous ; circumstances that seem to point 

 to the cultural treatment experience has 

 proved to be the best, viz., diffused light and 

 abundance of moisture. In Bifrenaria Harri- 

 soniae the structure is essentially the same, 

 and needs no detailed explanation further 

 than to note the numerous small fibro-vascular 

 bundles immediately above the lower 

 epidermis ; the leaf has thus a very 

 stringy appearance when torn. 

 This structure indicates no neces- 

 sity for direct solar exposure. 

 Lycaste Skinneri has a similar but 

 not an identical structure. Fig. 7, 

 Vanda tricolor. Like most mono- 

 podial Orchids, it comes from a 

 locality where the temperature is 

 always high and its fluctuations at 

 a minimum, and where the hygro- 

 metric conditions of the atmosphere 

 are for the greater part of the year 

 the most intense. Its leaf struc- 

 ture is thence adapted to these 

 conditions ; a thick cuticle and closely packed 

 epidermal cells and numerous underlying 

 elongated cells approaching the pahsade form 

 implying more or less exposure to a vertical 



