120 PLANTS OF NEW ZEALAND 



stomata by which the plant transpires. The object of this 

 arrangement is two-fold. It enables free transpiration to go on 

 in the wet season, when the plant is surrounded by moisture 

 that would otherwise choke the pores ; and, in the dry season, the 

 concealment of the pores checks excessive transpiration. Again, 

 Dendrobium and Earina very frequently grow upon trees, or in 

 the clefts of rocks. In such situations, of course, no moisture 

 can be received from the soil ; and if the leaves were thin and 

 broad, there would be excessive transpiration, and the plant 

 would wither. Consequently, they are narrowed, and have 

 become rather thick and leathery. The dimensions given in 

 Hooker's Handbook for the breadth of the leaves in these 

 genera are : E< mucronata J to i in., E. suaveolens $ in., 

 D. Cunninghamii J in. Contrast these leaves with those of a 

 shade-growing plant like Corysanthes macrantha. The latter 

 has round flaccid leaves full of moisture, which would soon 

 wither if exposed to strong sunlight. In Micro tis, the leaf 

 has been rolled up into a cylinder, no doubt with the object of 

 conserving its moisture, In Bolbophyllum, the leaf has been 

 reduced to a scale-like process, 



But Earina and Dendrobium, have not only endeavoured to 

 check transpiration through the leaves, they have also increased 

 the root surface, and specialized it for the absorption of 

 moisture, After clamping themselves to the rock, or to the bark 

 of a tree, by their roots, they produce numerous other white, 

 membranous, papery, filamentous roots. The outer surface of 

 these roots is composed of a spongy tissue, that cannot fail to> 

 absorb any moisture in the vicinity. 



Key to the Genera. 



1. Perennial epiphytes or rock plants. 2 



Roots terrestrial, stems annual. 6 



2 Pollen granular. 3 



Pollen waxy. 4 



3. Stem slender, leafy. Sepals free, Disk of lip 



naked. Earina p. 122, 



4. Leaves not on pseudobulbs . 5 



Leaves on pseudobulbs. Bolbophyllum p. 124.. 



