5 



II. EVOLUTION OF THE ORCHID FLOWER 



The flower in the Orchidaceje has been extraordinarHy modified from its original 

 form. Text-fig. 2 is a diagram of the flower of the Monandrs (all orchids except 

 the Diandra^). There are three sepals, two petals, and a labellum, or lip (an often 

 3-lobed development of the former petal. Pi). There 

 is only one anther (A') and two stigmas, the original 

 third stigma having been transformed into the rostel- 

 lum, a delicate mechanism for affixing the pollinia to 

 the heads of insects. 



Tliis flower has been derived from the typical yv^.. .„3 .3 /, 



flower of the Monocotyledons, which consists of five i^.-S'AV (p3) ^'^ //l.s. 

 whorls, each of three simHar organs, i.e. three sepals, X'^^^Jy^ 



three petals, an outer whorl of three stamens (^x,^^ Text-fig. .. u.s. Upper sepal, ls 

 and Ai in the diagram), an inner whorl of three ^^'^'^^' ^^^p^'- ^'' ^'- p^'^'^- -L. (p^J 

 stamens (a^, a^ and ^3), and three stigmas (Jx, ^. and p:e.ed1;;he^fof rterwhol'I^^T 

 ^-3) Evolution has resulted in the Monandry in the '^r::;i:^\^'.::Z:S' t' si 

 tollowmg changes: (i) The conversion of the petal ^"''''^ stigmas, usually confluent. ' r.' 



P3 into fl laKpIliit-n rvr i;t^ f^\-V\ ■ rr Rostellum, evolved from J'-. A\ A\ At. 



i 3 mtO a labellum or hp. (2) The suppression of five on same radii as sepals, «., «s ^3 on sLme 



out of the SIX anthers (the rudiments of two of which ""''" "' P''^'''' 



often appear as a little Icnob (staminode) on each side of the base of the colun^) 



(3) The conversion of the upper stigma S^ into the rostellum 



A still more important change, which probably preceded the others, is that the 

 filaments (stalks of the anthers) and style (stalk-like prolongation of the ovary on 

 which the stigmas are situated) have become fused together to form a sohd central 

 body called the column, or gynostemium, which is the outstanding feature of the 

 Orchidace^. In the course of these changes the regular flower of the Monocotyledons 

 has become so irregular that it can only be divided into symmetrically similar halves 

 from back to front down the middle (2ygomorphic). 



.1 \'^' ^T^T.'^T T '^"^ ^'"^' "^*^'^' '' ^"d ^" the only fertile anther in 

 he Monandra. (A^) has been changed into a large more or less petaloid staminode 

 (i 1. A fig. 4), which forms a sWeld sheltering the column, and probably a landing- 

 stage for msects leading to the entrance of the bag-like lip 



That the flower of the Monandry was originaUy regular is shown by the Australian 

 genus m/jm^ra (PI. 2, fig. i, T. venosa) with three sepals and tliree petals-an orchid 

 with regular flowers persisting to the present day-and by the very exceptional 

 reversion of the lip to a petal m various genera, e.g. PJatantbera Mfol^a, Orchis mono 

 O. ustu/ata, Ophrys aptjera, etc. (PI. 58 A). That a petal can develop into a lip is 



