October, 1916.] 



THE ORCHID WORLD. 



255 



Fig:. 2. Transverse section of leaf of Cattleya intermedia across 

 the mid-rib, X 30 diameters. 



1, upper, 2, lower epidermis: 3, fundamental tissue of soft thin- 

 walled cells containing chlorophyll granules : 4, fibro-vascular 

 bvmdle of mid-rib ; 6, smaller bundles. 



almost circular colourless cells filled with air 

 or water ; a similar layer of smaller and more 

 closely packed cells underlies the lower 

 epidermis ; the cellular tissue between them 

 consists chiefly of egg-shaped cells filled with 

 granules of green colouring matter, the 

 chlorophyll, not shown in the figure ; they are 

 less closely packed than the others, and have 

 interspaces here and there between them 

 which communicate with the outer air by 

 means of minute pores or stomata in the ^ 

 epidermis, also not shown in the figure. In 

 the centre is the mid-rib, and on each side of 

 it a smaller fibro-vascular bundle which ^ 

 contribute to the strengthening of the leaf, ^ 

 passage of fluids, etc. The leaf of Dendro- 

 bium nobile is thin, and its means for the 

 storage of water is limited, presumably there- 

 fore needing diffused rather than direct sun- 

 light, as under the influence of the latter, 

 evaporation would be too rapid. The leaves 

 of Dendrobium Jenkinsii are small and thick, 

 with three layers of cells called from their 

 peculiar appearance palisade cells underlying 

 the hypodermal water cells, and which are 

 always full of deep-coloured chlorophyll 

 granules, their numbers, size and intense 

 coloration being always in direct relation to 



the amount of light. When these palisade 

 cells are well developed it is a sure sign that 

 in their native country the plants arc exposed 

 to bright sunshine ; whilst on the other iiaiid 

 their abundant supply of chlorophyll enables 

 them to endure adverse conditions better than 

 less richly endowed plants can do. 



Fig. 2 shows a transverse section of a leaf 

 of Cattleya intermedia at the mid-rib. The 

 cells of the upper part of the fundamental 

 tissue are much elongated, approaching the 

 palisade form and are filled with chlorophyll 

 granules. The general structure compared 

 with that of the labiate Cattleyas is simpler, 

 thus affording another distinctive mark 

 between the two-leaved and usually long- 

 stemmed and the one-leaved short-stemmed 

 Cattleyas. It is a remarkable fact, too, that 

 similar sections of a leaf and stem of another 

 two-leaved Cattleya, C. velutina, proved to 

 be nearly identical in structure with those of 

 C. intermedia. 



Fig. 3, Cattleya Mossias. Here the minute 

 structure is more elaborate, presumably 

 indicative of the more changeable climatic 

 conditions under which the plant grows. The 



Fig. 3. 



Transverse section of leaf of Cattleya Mossis at the 

 mid-rib. 



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

 sometimes called the mesophyll. containing chlorophyll granules 

 (not shown in figure); the layer immediately under the upper 

 epidermis indicated by 3a and called the hypoderm. consists of 

 colourless cells containing water : 4, air ca\ ities : 5, fibro-vascular 

 bundle of mid-rib shown diagrammatically. the upper club-shaped 

 portion consists of thick w^oody cells which serve to protect the 

 more delicate tissue beneath ; 6, smaller bundles (veins', running 

 parallel with mid-rib; 7. still smaller bundles(veinlcts\ 



