ORCHID 



ORCHID 



1163 



is continued by an axillary bud originating in the axil of 

 one of the leaves. The whole plant is thus built up 

 of branches sympodially united. The lower part of 

 each new axis is prostrate at first and bears only scales. 

 It is known as the rhizome. Later the apex turns 

 upward and bears ordinary leaves. In many the erect 

 portion of the stem becomes thickened into a food 

 reservoir known as a pseudobulb. The pseudobulb 

 itself may consist of several internodes, as in Laelia, 

 Cattleya. It is then clothed with leaves, at least when 

 young, and bears the scars of the fallen leaves. In 

 other cases only a single internode is thickened. This 

 bears 1 or 2 Ivs. at the summit, but has no leaf-scars. 

 The new shoot which continues the growth of the plant 

 arises in the axil of one of the scales below the pseudo- 

 bulb. The manner of growth is shown in Figs. 1566 and 

 1567, in which the parts marked a a and b b, respectively, 

 represent the growth of a branch with its basal portion 

 or rhizome and the terminal portion or pseudobulb. In 

 the terrestrial Orchids pseudobulbs are usually not 

 formed and the erect portion is a long or short stem 

 clothed with leaves (Sobralia, Selenipedium) . The in- 

 florescence is either axillary (Dendrobium, Fig. 1566) 

 or terminal (Cattleya, Fig. 1567). 



The habit of the plants depends in a great measure 

 upon the rhizome. When this is long the plants are 

 loose and straggling, and when it is short they are com- 

 pact in habit. In some the rhizome becomes suberect or 

 climbing (species of Lycaste). 



aba 

 1567. Cattleya. 



The light and shaded parts, a a and b b, each represent 

 one year's growth. 



The Orchid flower exhibits perhaps the greatest 

 specialization and adaptation found anywhere in the 

 vegetable kingdom. The 2 outer whorls of floral organs, 

 the sepals and petals, have been sufficiently described. 



In the monocotyledons there are normally present 2 

 whorls of stamens. In the Orchids only 1 or 2 of these 

 are fertile. In the Monandrae (Cypripedium, etc.) the 

 odd stamen of the outer whorl is developed into a wing- 

 like staminodium (Figs. 1568, 1569, st). The similar sta- 

 mens of the inner whorl are fertile, and form 2 anthers, 

 1 on each side of the staminodium (Figs. 1568, 1569, a). 

 In all the other Orchids, except in abnormal cases, only 

 the odd stamen of the outer whorl is fertile, bearing an 

 anther situated at the top of the column (Fig. 1570, a). 

 The similar two of the inner whorl are developed as 

 staminodia, forming the sides of the clinandrum or 

 anther-bed. Often they are developed into crests or ears 

 on the column. Traces of the other stamens are rarely 

 found in the flower. Compare Figs. 642-644, Vol. I. 



The 3 pistils are developed in Cypripedium and a few 

 related genera. In these the stigma is clearly 3-lobed, 

 showing the union of 3 pistils (Figs. 1568, 1569, s). In 

 most of the other genera only 2 of the stigmas are 

 receptive, the third being developed into the curious 

 rostellum. In some cases the stigmatic surfaces are 

 confluent into one, while in others they remain more or 

 less distinct. Generally they appear as flat surfaces 

 often sunken in a depression in the column (Lseliineee, 

 Fig. 1570, s, and many others). In a few cases the 

 sti<j:'iias are more or less elevated on stalks (Habena- 

 riai. In Sophronitis they extend partially along two 

 wing-like projections of the column. 



The odd pistil, the rostellum, is situated above the 



stigmas, separating them from the anther-bed. In the 

 Lffiliinera its lower surface is still continuous with the 

 stigmatic surface (Figs. 1570, A, B, r). The principal 

 function of the rostellum is the secretion of a viscid 

 fluid, by means of which 



the pollen masses adhere to \ .^ s ^ 



insects visiting the flowers. 



1568. Cypripedium. 



st, staminodium; a, anther; 



s, stigma. 



1569. 



Selenipedium caudatum. 



st, stain medium; a, anther; 



s, 3-lobed stigma. 



The anther lies above the rostellum, within a depres- 

 sion or anther-bed (Fig. 1570, a). Its cells vary from 

 2-4 or 8. In nearly all the Orchids the pollen cohere? 

 in masses or pollinia (Fig. 1570, p), the number oi 

 pollen masses corresponding to the number of anther 

 cells. Often the viscid substance uniting the pollen 

 grains is prolonged into a stalk (caudicle), which ex- 

 tends beyond the anther and comes into contact with 

 the viscid substance secreted by the rostellum, which 

 forms an adhesive disk by means of which the pollinia 

 become attached to insects. In many Orchids the outer 

 layer of the rostellum itself separates by a dissolution 

 of the underlying cells, and thus forms a stalk (stipe), 

 which becomes attached to the pollinia by means of the 

 product of the dissolved cells. Whatever its origin, 

 the stalk, with its viscid disk, forms one of the most 

 important parts of the mechanism by means of which 

 pollen is transported from flower to flower, insuring 

 cross-pollination of the group. The details of the 

 mechanism by which this is accomplished have been 

 beautifully explained by Darwin in his classical work, 

 "The Various Contrivances by which Orchids are Fer- 

 tilized by Insects." 



The relation of the parts of the flower to one another 

 is often greatly changed by the peculiar growth of the 

 floral axis. This is convex in the very young stages of 

 development, but it soon becomes cup-like and finally 

 tubular, inclosing the ovary. Special lateral outgrowths 



B. Section through the col- 

 Column of Cattleya. umn of Cattleya. 

 a, anther ; r, rostellum; p, pollinium; r, rostellum; 

 *, stigma *, stigma 

 1570. Details of the column of Cattleya. 



near the top of the ovary form the "foot" of the column 

 found in many orchids (Pescatoria, Phaius). When the 

 foot is present the labellum is attached to its apex, and 

 often the sepals are decurrent upon it, forming a men- 



