2386 



ORCHIDS 



ORCHIDS 



partitions or cross-walls, into four or eight cells. In a 



few cases, six cells are formed, probably by the abortion 



of some of the cells. 



The pollen is powdery in some cases, but usually the 



grains are united by means of a viscid substance into 

 waxy masses termed "pol- 

 linia." The number of pollinia 

 corresponds to the number of 

 anther-cells. In many genera, 

 the viscid substance uniting 

 the pollen-grains is prolonged 

 beyond the pollen-mass and 

 hardens into a stalk or 

 "caudicle" within the anther- 

 cell (Fig. 2651). One or two 

 pollen-masses are attached 

 to each caudicle. When the 

 anther dehisces, the caudicles 

 come into contact with 

 masses of sticky matter 

 formed by the disorganiza- 

 tion of cells on the upper 

 surface of the rostellum. In 

 many orchids, this substance 

 itself forms the adhesive disk 

 by means of which the pol- 

 linia become attached to 

 insects. In others a strip of 

 tissue is separated from the 

 rosteUum by the disorgani- 



JT <* 4 underlying 

 cells, and forms a stalk 

 which in this case is known 

 as the "stipe." When a stipe is formed by the 

 rostellum, the caudicle of the pollen-masses is not 

 produced. In that case, the poUinia become directly 

 attached to the stipe when the anther dehisces. The 

 other end of the stipe forms the adhesive disk. (Fig. 

 2652.) Whatever its origin, the stalk with its adhesive 

 disk is one of the most important parts of the mechanism 

 by which the transport of the pollen-masses from flower 

 to flower is brought about and by means of which cross- 

 pollination is insured. The details of the process by 



2637. Flower of Pogonia, 

 a pretty native orchid. 

 Beneath the expanded part 

 is the long ribbed ovary ; 1, 2, 

 j, sepals; 4, 5, 6, petals, one 

 of them (5) enlarged into a 



uralsize.) 



2638. Diagrams of orchid flowers in cross- 

 section. A, Monandrae (one-anther) and B, 

 Diandrae (two-anthers) ; I, represents the label- 

 turn, and oo (in the inside of the diagram) the 

 staminodia. 



2639. 



Orchid flower, 

 showing the 

 long stem-like 

 ovary between 

 the two bracts 

 and the other 

 floral parts 

 above. 



which this is accomplished have been 

 beautifully explained by Darwin in his 

 classical work, "The Various Contriv- 

 ances by Which Orchids are Fertilized 

 by Insects." 



Dimorphic and trimorphic flowers occur hi a few 

 orchids. In the inflorescences of a small group of 

 Oncidiums (Heterantha), only few normal flowers are 

 produced together with many smaller sterile ones which 

 lack the column. In Renanthera Lowii the upper 

 flowers of the long inflorescence differ in form and color 

 from the others. The most unusual condition exists in 

 Catasetum, in which there are three very different 

 types of flowers. In some years the same plant pro- 

 duces only one type and in other years all types on a 

 single inflorescence. 



2640. Orchid ovaries in cross-section. 

 Paphiopedilum Charles worthii at left; P. 

 Schlimii at right. 



The flowers of orchids are commonly noted for their 



longevity. When not pollinated, the flowers of most 



genera remain fresh for thirty to forty days, and in 



some genera even for much longer periods. Only a small 



number of 



genera have 



flowers that 



wither in a few 



days (Sobralia, 



Cirrhopetalum) . 



The persistence 



of the flowers 



greatly increases 



the chances for 



pollination, 



which, since it is 



dependent on particular agencies, is more or less uncer- 

 tain. After pollination has been effected, the flowers 



soon wither. 



The fruit of orchids is usually a dry capsule, requiring 



a long time to ripen, so that if an orchid flower is fer- 

 tilized in one rainy season its 

 seeds are not disseminated until 

 the next wet period. Very few 

 have fleshy fruits which dehisce 

 imperfectly or not at all. In them 

 the seeds are liberated only by 

 the decay of the pericarp (Vanilla) . 

 The seeds of orchids are minute 

 and extremely numerous, the num- 

 ber in a single capsule having 

 been estimated for different species 

 from several thousands to over a 

 million. The seeds consist of a 

 dry loose large-celled testa inclos- 

 ing a rudimentary embryo, which 

 in most genera is entirely un- 

 differentiated. (Fig. 2653.) 



Symbiosis. 



So far as known, the roots of 

 all orchids, with the exception of 

 occasional individuals, are in- 

 habited by fungi which live within 

 the tissues either symbiotically or 

 at least without causing any 

 apparent injury to the plant. The 

 fungus hyphse are primarily found 

 in the cortical tissues, but usually 

 two or three of the outer layers 

 of cells of the cortex are not in- 

 fected. In the epidermis the fun- 

 gus is found only in the cells 

 bearing root-hairs, through which 

 long hyphae pass out into the substratum. The central 

 cylinder and the growing apex of the root are not 

 infected. The extent to which the fungus passes upward 

 in the plant varies with 

 different species and with 

 the condition of the roots. 

 In Neottia, Goodyera, and 

 Corallorhiza the lower 

 part of the stem is invaded 

 sometimes up to the first 

 node. InEpipactis, Cypri- 

 pedium, and Listera, the 

 roots and rhizomes con- 

 tain much woody tissue. 

 In such forms, many of 

 the roots are free from 

 fungi, and in the remain- 

 ing roots, as well as in the 

 rhizomes, the distribution 

 of the fungus is irregular. ^ a longitudinal section y a t 

 The basal parts of the th e right. a, anther; r, rostel- 

 roots, which are more or lum; s, stigma; p, pollinium. 



2641. Stanhopea 

 platyceras. A. com- 

 plete flower. B. sec- 

 tion, Is. lateral sepals; 

 os, odd sepal; lp, lat- 

 eral petal; h to op, 

 odd petal or labellum 

 of which h is the hy- 

 pochil, m the meso- 

 chil, and op the epi- 

 chil, ms, mesidium, 

 and x, pleuridia. 



