150 



KNOWLEDGE, 



[July 1, 1899. 



growing on trees ; these often produce aerial tubers, and 

 have long pendant roots, which do not, however, establish 

 any connection with the soil. A few are climbers, others 

 grow on rocks, but the majority are terrestrial like our 

 British species, which abound in damp meadows. 



The perianth of the lilies, irises, and daffodils, is regular, 

 all the parts in each whorl being of similar size and shape ; 

 orchids, on the other hand, have a highly irregular peri- 

 anth. Notwithstanding the eccentricities" of their shapes, 

 however, the flowers of orchids are comparatively simple 

 in structure, and may be regarded as merely modifications 

 of the ordinary lilaceous type, which is made up of five 

 alternating whorls of five parts each. The irregularity of 

 the perianth is mainly due to the extraordinary develop- 

 ment of the labellum or lower lip. This organ is formed 

 by the combination of one of the inner segments of the 

 perianth with two sterilized stamens of the outer stamiual 

 whorl, its composite nature being usually indicated by its 



FiQ. 2.— Floral Diagrams: 1, Lily; 2, Oreliis ; 3, CTpripedium. 



three lobes. The labellum is frequently more highly 

 coloured than the other parts ; in many species it is pro- 

 longed into a hollow spur behind ; in others it is tongue- 

 shaped ; while in the slipper orchid it forms a concave 

 pouch or cup in front of the flower. At first the labellum 

 is superior, but in many species, by the twisting of the 

 ovary, the flower is inverted so that the labellum comes to 

 be available as a landing-stage for visitors. Sometimes it 

 is divided by contractions into three regions, which are 

 distinguished as the hypochilium, nest the base, the 

 mesochUium and epichilium. 



Of the six stamens, which, theoretically, the flower 

 ought to possess, only one is fully developed, the others 

 being represented by abortive structures termed stam- 

 modes. The single fertUe stamen is adherent to the style, 

 a condition described as gynandrous, and the combined 

 structure constitutes the column. The pollen contained in 

 each of the anther-lobes coheres into a waxy mass or 

 pollinium, the stalk of which is furnished with a viscid 

 disk at its lower extremity. The only other flowers in 

 which pollen-masses of this description occur are the 

 Asclepiads, an order of Dicotyledons, having no affinity 

 whatever with the Orchidaceie. 



A characteristic structure in orchid flowers is the 

 rostellum, which in our commoner species is a little knob 

 projecting from the top of the column, just above the 

 entrance to the spur of the labellum. It appears to be one 

 of the three stigmas of the original type, metamorphosed 

 in a remarkable manner. The rostellum consists of a 

 membraneous pouch containing two little rounded disks of 

 sticky matter which are attached to the bases of the two 

 masses of pollen. When any object touches it, the mem- 

 brane of the rostellum ruptures, and the sticky disks are 

 exposed, whereby the pollen-masses are glued to the object. 

 This can easily be verified by inserting a pencil into the 

 flower; on withdrawing it the pollen-masses will be seen 

 to have fastened themselves to the pencil by their sticky 

 glands. 



On the lower side of the column, immediately behind 

 the rostellum, the two functional stigmas are situated; 

 they are merely flat viscid areas, to which pollen readily 

 adheres, and may be more or less confluent from the line 

 of division becoming obliterated. 



The early spring orchis, 0. mascula, may be taken to 

 illustrate the mode in which cross-fertilization is aocom- 

 phshed. Humble bees are its most frequent visitors, and 

 the flower is specially adapted to their visits. Alighting 

 on the labellum the insect thrusts its tongue into the 

 hollow spur, the mouth of which is wide enough to admit 

 of this being easily done without touching the rostellum. 

 But the insect's head is too large to enter the spur, which is 

 longer than the bee's proboscis ; in the effort to reach the 

 bottom of the spur the visitor pushes its head against the 

 rostellum, with the result that the two pollen packets adhere 

 to its forehead, and are removed when the insect withdraws 

 from the flower. At first the pollinia stand erect, but in 

 the course of thirty seconds or so they spontaneously 

 curve into a horizontal position, and project in front of 

 the bee's head. If now the insect should enter another 

 orchid the pollina are so placed that they must inevitably 

 be pressed against the viscid surface of the stigmas, on 

 which one or both may be left adhering. Generally a 

 portion only of the pollen 

 is left on the stigma, the 

 remainder being avail- 

 able for other flowers 

 which the insect may 

 subsequently visit. The 

 stigmas being placed be- 

 low the anther, the poUen- 

 masses, if they retained 

 their erect position, could 

 not be delivered at their 

 proper destination ; in- 

 stead of being transferred 

 to the stigma they would 

 be pressed by the insect 

 against the anther of 

 the second visited flower. The automatic bending of the 

 poUen-masses is thus a provision for insuring their proper 

 delivery. It is a rather remarkable circumstance that in 

 the genus Orchis the spur never contains any nectar ; not 

 without reason therefore have the various species of orchis 

 been described as "sham-nectar producers," or flowers 

 which practice deception upon their visitors. It has been 

 shown, however, that insects pierce the succulent tissue 

 which lines the interior of the spur, and that probably 

 from its cells they extract a sweetened sap. A bee at 

 work on the early purple orchid has been observed to 

 spend three or four seconds in each flower- bell, which is 

 long enou,t;h to allow the cement of the sticky disks to 

 harden sufficiently ; two seconds or so are spent in passing 

 from one flower to another ; and as the visitor rarely 

 remains more than twenty seconds on one spike, it has 

 passed to another spike before the pollinia belonging to 

 the first have assumed the horizontal position. The cross- 

 fertilization is thus insured not merely of separate flowers 

 but of distinct individual plants. 



The marsh orchis, 0. latifolia, and the spotted orchis, 

 0. maculata, two of the commonest species, are fertilized 

 very much in the same way as 0. mascula. Their spurs 

 are slightly shorter, however, and, perhaps, for this reason 

 the spotted orchid is frequented by flies fully more than 

 bees. The fragrant orchid, Oi/ninaih'mia conopsea, has a 

 much longer spur than any of those previously named ; its 

 flowers are visited by numerous moths and butterflies, the 

 abundant nectar being only accessible to Lepidoptera. 



Fig. 3. — Diagrammatic Section of 

 Orchid : a, anther : r, rostellum ; s, 

 stigma ; p and q, pollinia ; r, pollinia 

 of 0. pyramidalis, with saddle- 

 shaped disk. 



