POLLINATION 19 



ment of the pollinia by Herminium and Ccsloglossum, but the actual line of descent 

 remains obscure. The same is the case with Ophrys, perhaps the most astounding 

 conception of all, based on the mimetic resemblance to the females of a particular 

 genus or even species of Hymenoptera, the climax of specialisation. 



It is not intended to imply that orchids are descended from existing species of the 

 Apostasiacea^, but merely that these indicate probable lines of descent from some 

 closely allied ancestral forms. Nemviedia suggests the facility with which the forldng 

 of the Orchidaceas into their two great sub-families may have been brought about 

 by the reduction of anthers to staminodes, a proceeding of undoubted occurrence 

 in the Orchidaceje. It must be admitted that the basic idea of the Apostasiaceae differs 

 from that of the Orchidaceas, but if creation has taken place through evolution 

 existing forms must have arisen from ancestors differing from them in conception 

 as weU as execution. 



IV. POLLINATION AND FERTILISATION 



Pollination simply means the placing of pollen on the stigma of a flower, and 

 is the first step towards fertilisation. When a pollinium brought by an insect 

 touches the stigma, some of the pollen adheres to the tenaciously sticky secretion 

 with wliich the stigma is covered, the puU of which is sufficient to break the 

 threads by which the pollen-grains or packets are bound to the pollinium, but not 

 as a rule sufficient to detach the whole pollinium. The insect goes on to other flowers, 

 distributing pollen on various stigmas, till nothing but the stump is left. 



Under the stimulus of the stigmatic secretion the packets of pollen disintegrate 

 into the tetrads (groups of four grains) of which they are composed, and finally as 

 a rule into single grains. Each grain puts forth a pollen-tube, which grows down 

 like a root into the ovary tiU it reaches an ovule (a seed in its earliest stage) which it 

 enters by a microscopic aperture called the micropyle, and there discharges its proto- 

 plasmic contents. This process is called fertilisation, and without it the ovule can 

 never develop into a fertile seed. The pollen-tubes enter the ovary by a passage called 

 the stigmatic canal, and are united into a bundle by a mucilage formed by the cells 

 of its walls, and convey the protoplasm with its nuclei to the ovules. Partitions are 

 formed in the pollen-tubes behind the protoplasm, in long tubes very numerous 

 ones.' The time required for the germination of the pollen-tubes is sometimes only 

 2-3 days {Listera ovata), 5-6 days in many species of Orchis, 9-10 days in Ophrys, 



' Camus, Icon. p. 73. 



J-2 



