2 ORCHIDS OF JAMAICA 



pseudobulb is of very various shapes, sometimes globose, 

 usually more or less elongated ; its surface is generally smooth 

 and shiny. In Pleurothallis and allied genera the shoots are 

 not thickened to form a pseudobulb, and bear a single leaf 

 "which spreads at right angles to the axis or apparently forms 

 a continuation of it. 



The leaves are simple, generally sessile, and wither and 

 decay on the stem, or, as in most epiphytic species, separate 

 by a distinct joint. The margin is entire and the apex often 

 denticulate or unequally cut ; the venation is as a rule parallel. 

 In some cases the blade passes gradually into a stalk which 

 has generally a sheathing base. 



At the basal nodes of the shoots leaf-scales are borne, which 

 pass gradually into foliage leaves, or the latter may start abruptly. 

 The leaves ^re generally arranged in two ranks, one each side of 

 the stem. 



Leaves of epiphytic forms are generally thick and fleshy ; 

 and by a thick cuticle they are adapted for storage of water. A 

 main root is never present ; its place is taken by adventitious 

 r<Jots which arise especially from the nodes and often show as 

 regular an arrangement as the leaves. In terrestrial orchids the 

 t'oots are sometimes thickened to form stores of reserve material. 

 The air-roots of epiphytic orchids have a special development of 

 the outer layers forming a sponge-like tissue {velamen), by which 

 water is absorbed for the plant from the atmosphere. Three 

 kinds of air-roots may be distinguished. Clinging roots which 

 creep close to the substratum and are inseparable from it; 

 absorptive roots, or branches of the former which grow into the 

 huinus which collects about the plant ; and the fine aerial roots 

 which hang down sometimes to a considerable length. 



The flowers are hermaphrodite and irregular. The perianth 

 consists of an outer whorl of three sepals and an inner whorl of 

 three petals which alternate with the sepals. The odd or median 

 sepal is normally on the lower side of the flower and furthest 

 from the axis. The odd petal, which generally differs con- 

 siderably in size and shape from the two lateral petals and is 

 termed the lip, is normally on the upper side of the flower next 

 the axis. This position is, however, generally reversed by a 

 twist of the ovary so that the lip in the open flower is on 

 the lower side. The sepals are usually smaller and less con- 

 spicuous than the petals, but in some genera (as Pleurothallis, 

 Lepanihea and Brassia) they are much larger. They are free from 

 each other or more or less coherent ; in some genera, as Pelexia and 

 Stenorrhyncos, the lateral sepals are united at the base to form a 

 spur which in Pelexia encloses the elongated base of the lip. 

 The petals show a great variety in form and colour ; the lateral 

 pjtir are generally smaller than the lip. The lip which is, as a 



