51 



does not press against the sticky surface of the disc. It is 

 only on retiring (and this process is made a trifle difficult owing 

 to the direction of the hairs) that it will come in contact with 

 the adhesive part of the disc, which it will then carry away 

 in a position vertical to its own body. But when the pollinia 

 perform the ''act of depression," the pollinarium becomes 

 horizontal, or almost so. When the next flower is visited the 

 pollen masses are deposited in their respective cups in the stig- 

 matic cavity, and the visitor departs bearing with it the disc 

 and the stipites. 



I do not knoAv what the actual fertilizing agent is in the 

 case of Dipodium, but judging by the size of the landing place 

 and the strength of the labellum it is probably an insect of 

 fair size. 



Orthoceeas strictum, Br. PL vii.B. 



The conformation of the flower of this species is shown in 

 flg. 1, pi. vii.B. The dorsal sepal (d s) is relatively large and 

 hooded, enclosing within it the column and diminutive lateral 

 petals. The linear lateral sepals (I s) spread horizontally on 

 each side. The labellum fl) is thick, fleshy, and trilobed, the 

 middle lobe greatly exceeding the lateral ones in size. In the 

 mature flower the labellum lies closely reflexed against the 

 ovary (ov), and at its base there is exposed a large yellow gland 

 with a horizontal groove on its posterior surface. 



It will be convenient to study the mechanism of pollination 

 at several stages in the development of the flower. 



1 . In the advanced bud the lateral sepals are parallel and 

 vertical, and the front of the hooded dorsal sepal is completely 

 closed by the labellum, which is also at this stage vertical. 

 The first indication that the flower is about to expand is given 

 by a slight divergence of the lateral sepals and a slight descent 

 of the labellum. If the column be examined at this stage 

 (fig. 2) it will be found that the stigma is almost oval in shape, 

 the short diameter being placed horizontally. Its anterior sur- 

 face is glistening, concave, and at the base there is a well- 

 marked funnel-shaped depression which penetrates almost, but 

 not quite, into the stigmatic canal. Posteriorly the stigma 

 presents a flat surface with a slight triangular mesial ridge, 

 which terminates at the base of the rostellum. A transverse 

 section of this ridge exposes a cavity continuous with the stig- 

 matic canal. 



The rostellum (r) is slightly lower than the anther-tip (a) 

 and is placed vertically. 



The lateral appendages (I ap) of the column reach a little 

 higher than the rostellum, but not quite as high as the anther- 

 tip. Their free ends incurve towards each other. They are 

 partly hidden by the lateral margins of the stigma, and are 



