2382 



ORCHIDS 



ORCHIDS 



Morphology of the vegetative parts. 



All orchids are perennial herbs which, according to 

 their mode of growth, fall into two groups the mono- 

 podial and the sympodial. 



In the monopodial orchids, the growth of the main 

 stem is continued indefinitely by the terminal bud. 

 (Figs. 2624-2627.) Lateral branches are frequently 

 produced, but they do not regularly assume the part of 

 the main axis and do not under ordinary conditions 

 exceed it in length. The growth of the main stem and 

 its branches may be interrupted for a tune by a period of 

 rest, but such interruption is not manifested on the stem 

 by the formation of scales and juvenile leaves when 

 growth is resumed. All leaves are similar. The habit 

 of the monopodial orchids is various. In spite of their 

 indeterminate growth, they do not usually attain great 

 size. This fact is explained by the slow growth of 

 the plants, which often require several months to 

 develop a single leaf. Nevertheless, some of the species 

 attain stately proportions. The climbing species of 

 Renanthera sometimes reach a length of 12 feet or 

 more in greenhouses. Some of the vandas, aerides, and 

 angracums form handsome plants 1 to 2 feet high and 

 well clothed with long distichous leaves. (Figs. 2626, 

 2627.) While these forms grow into tall leafy plants, 

 others, like Phalaenopsis, remain almost stemless and 

 possess only a few large fleshy leaves. The extreme 

 reduction is represented by such forms as Polyrrhiza 

 funalis, in which the stem is reduced to a mere scaly 

 bud seated on a mass of tangled green roots which 

 perform the function of photosynthesis. 



In the sympodial orchids, the growth of each shoot is 

 definitely terminated usually after one, rarely after two 

 or more seasons. The development of the plant is con- 

 tinued by buds originating in the axils of the scale- 

 leaves at the base of the parent shoot. (Figs. 2628, 

 2629.) The lower part of each new axis is prostrate at 

 first and often subterranean and bears only scales. It is 

 known as the rhizome. In many terrestrial orchids of 

 the temperate regions, the lower terminal part of the 

 rhizome forms a tuberous root from whose apex the 



shoot of the next season arises. (Fig. 2630.) In a few 

 sympodial orchids with perennial stems and climbing 

 habits, branches originate in the axils of the stem- 

 leaves (Vanilla). These bear a strong resemblance to 



2626. Angraecum superbum, to show a monopodial growth. 



2627. Vanda tricolor, a branching monopodial orchid. 



the monopodial orchids, but all the branches as well as 

 the main stem are finally terminated by inflorescences. 



The general habit and appearance of orchid plants 

 depends to a great extent on the nature of the rhizome. 

 When this is long, the plants have a loose straggling 

 habit (Epidendrum). When it is short, the plants are 

 compact (Masdevallia, Cattleya, Ccelogyne). The 

 rhizome may be much branched and give rise to numer- 

 ous upright shoots (Sobralia, Coelogyne, Disa), or it 

 may be simple and give rise to only one shoot annually 

 as in many of our native orchids. In some species, the 

 rhizome assumes suberect or climbing habits (Lycaste). 



The upright part of the axis presents a great diversity 

 of forms which may be grouped in two classes according 

 to the position of the inflorescence. This may be either 

 terminal or lateral. 



In the forms with a terminal inflorescence, the 

 leafy part of the stem may be very short so that the 

 leaves appear as a rosette on the ground while the upper 



Eart of the stem bears bracts from whose axils the 

 owers appear (Goodyera, Orchis rotundifolia, Cypri- 

 pedium acaule), but generally it is more developed and 

 bears a succession of leaves which are gradually reduced 

 to bracts in the upper part (Habenaria, Cypripedium. 

 Disa, Thunia). In Selenipedilum and Sobralia, the tall 

 reed-like stems reach a height of 6 to 15 feet and often 

 form dense thickets. In Vanilla the stems are long, 

 branched, and climbing. In many of the epiphytic 

 forms the stem is thickened into a pseudobulb. This 

 may consist of a single internode which bears one or 

 two leaves at its summit (Coelogyne), but more fre- 

 quently several internodes are enlarged to form the 

 pseudobulb, which is then clothed with leaves at 

 least when young, and later bears the scars of fallen 

 leaves (Lselia, Cattleya, Epidendrum). Figs. 2631 

 and 2632 show the two forms of pseudobulb. Some- 

 times the whole stem is more or less fleshy without 



