224 RECEPTION OF FLOWER-SEEKING ANIMALS AT THE FLOWER. 



below (fig. 266^); later, the rachis becomes pendent, and the apex of the inflores- 

 cence points downwards. The buds are now so placed that the standard is below. 

 Before the standard unfurls and the flowers become accessible, however, the stalk 

 of each flower twists round through nearly 180°, so that the standard is again 

 brought uppermost (c/. fig. 256 ^). In this position the keel is a convenient plat- 

 form for visiting insects to alight upon. It is of interest to note that if the young 

 racemes are forcibly retained in the erect position by means of strings, the twisting 

 of the fiower-stalks does not occur, or only to a very slight extent. In the nearly 

 allied Laburnum Alschingeri the racemes are always borne erect, and there is no 

 twisting of the stalks at all; by this character the two species may be readily dis- 

 tinguished. 



Many examples of the same phenomenon are furnished by Orchids. Here, 

 however, the twisting is accomplished not by true flower-stalks but by the stalk- 

 like inferior ovaries. In Orchid flowers one member of the perianth, the lip or 

 labellum, is conspicuous by its shape and size, and serves in more than two-thirds of 

 all Orchids as a landing-stage for insects. This petal is directed upwards in the bud, 

 and in a few Orchids always retains this position, as in the vanilla-scented Nigri- 

 tella and in JSpipogium (cf. fig. 257 ^''). But in the majority of Orchids, such, for 

 example, as grow in the meadows with erect spikes of flowers, the ovary undergoes 

 a spiral twisting which brings the lip below so as to serve as a platform for the 

 insects. Nor is this confined to our indigenous Orchids; it occurs also in tropical, 

 epiphytic forms which grow perched on branches of trees or ledges of rock, when 

 they have an upright rachis to the inflorescence; as, for instance, in Oncidiwm 

 Papilio, represented in the accompanying Plate XIII., entitled, " West Indian 

 Orchids". Many of these epiphytic Orchids, however, have not ascending but more 

 or less pendent inflorescences; this is markedly the case in Stanhopea, of which a 

 species, Stanhopea Devoniensis, is represented in the plate beside the Oncidiwm. 

 Such flowers do not require to twist to bring the lip into the position in question. 

 Indeed, in this and many other similar Orchids no twisting of the ovary takes place. 

 If, however, a young spike of Stanhopea be fixed in an erect position, the flowers 

 will all twist within twenty -four hours and take up the position which they would 

 have occupied had the inflorescence been pendulous. 



Altogether peculiar is the state of aflfairs in Gongora galatea, a tropical 

 American Orchid sometimes introduced into European hot-houses. As in Stan- 

 hopea, the spikes are pendulous from the branches of old trees, but the lip of the 

 flower in its original position below is unsuited as a platform for insects. Conse- 

 quently the ovaries twist through 180°, so that the lip stands above and is of service 

 to visiting insects. 



It is an interesting circumstance that all the flowers on many erect, flowering 

 axes turn towards the same side, so that a one-sided spike or raceme results, as in 

 Vicia, Digitalis, Corydalis, and Penstemon. The entrance to the flowers is directed 

 towards the side from which the visits of insects or humming-birds may he 

 expected. When, for instance, a Foxglove (Digitalis) grows by the edge of a wood. 



