PHYSIOLOGY 



287 



some oases (Linurn perenne) legitimate fertilisation alone is productive. Legitimate 

 fertilisation is rendered more certain by the fact that insects in visiting the flowers 

 touch correspondingly placed sexual organs with the same portions of their body. 

 The flowers of Primroses have styles of two different lengths (dimorphic hetero- 

 styly) ; the same peculiarity is exhibited by Pulmonaria, Hottonia, Fagopyrum, 

 Linum. There are also flowers with tuimorphic HETEROSTYLY(ij/</w'ttm Salimria, 

 and some species of Oxalis), in which there are two circles of stamens and three 

 variations in the height of the stigmas and anthers. 



In a great number of flowers self-pollination is made mechanically impossible, 

 as their own pollen is prevented by the respective positions of the sexual organ 

 from coming in contact with the stigma (Heroogamy). In the Iris, for example, 

 the anthers are sheltered under the branched petaloid style, upon whose lip-like 

 stigma no pollen can come, unless through the agency of insects. In the Orchid- 

 aecae and Asdepiadaceaa self-pollination is rendered impossible both by the nature 

 of the pollen masses and by their position. A complicated form of structural con- 



Fig. 219. — Pollination of Salvia prutcnsis. 1, Flower visited by a bumble-bee, showing the projec- 

 tion of the curved connective from the helmet-shaped upper lip, and the deposition of the 

 pollen on the back of the bumble-bee ; 2, older flower, with connective drawn back, and 

 elongated style ; 4, the staminal apparatus at rest, with connective enclosed within the upper 

 lip ; 3, the same, when disturbed by the entrance of the proboscis of the bee in the direction of 

 the arrow ; /, filament ; c, connective ; s, the obstructing half of the anther. 



trivance, by means of which cross-pollination is secured, may be seen in a flower 

 of Salvia pratensis (Fig. 219). The anthers of this flower are concealed in the 

 upper lip of the corolla, from which the style, with its bilobed stigma, projects. 

 "When a bumble-bee visits the flower in search of honey, it must first with its 

 proboscis push out of the way the small plate (s), formed of two sterile anther 

 halves grown together. These are situated at the ends of the short arms of the 

 connectives (a), which are so elongated that they might easily be mistaken for the 

 filaments (/) of the stamens. The fertile anther halves are situated at the other 

 ends of the connectives, and so are brought in contact with the hairy back of the 

 bumble-bee when it pushes against the plate at the short ends of the lever-like 

 connectives. The pollen thus attached to the bee will be brushed off its back by 

 the forked stigma of the next flower it enters. Good examples of hercogamous 

 flowers are afforded by the Papilionaceae, by Kalmia, whose anthers are held in 

 pockets of the corolla, by Vinea, Aristolochia, etc. 



Hybridisation. — The union of two sexual cells is, as a rule, only 

 possible when they are derived from closely allied plants ; it is only 

 then that they exercise an attractive influence upon each other and 



