28o REPRODUCTION 



number of flowering plants are cross-fertilised, and experiments 

 have proved that the plants derived from seeds which have 

 arisen from cross-pollinated flowers are in many cases taller, 

 more robust in constitution and productive of earlier flowers and 

 more seeds than those arising as the result of self-fertilisation. 



A great many devices are met with among flowering plants 

 which are calculated to secure a preponderance of cross-fertilisa- 

 tion over self-fertilisation. The chief arrangements tending more 

 or less completely to this end are the following : — 



(i) The flowers are often diclinous (p. 87) ; that is, the 

 sexual organs are produced in separate flowers, which may occur 

 either on the same plant, as in the hazel and pine, or upon 

 different individual plants, as in mercury (Mercurialis), hop and 

 willow. 



(ii) Although the male and female sexual organs in 

 monoclinous flowers are in close proximity to each other, they 

 frequently do not ripen together : plants bearing flowers of this 

 kind are described as dichoganwus. 



Two types of flowers are met with upon dichogamous plants, 

 namely, ia) protandrous flowers, or those in which the anthers 

 ripen and shed their pollen before the stigma is in a suitable 

 condition to receive it, and {b') protogynous flowers in which the 

 stigma is receptive some time before the anthers open and set 

 free their pollen. 



Protandrous flowers are abundant ; the sunflower, daisy, dead- 

 nettle, carrot, bean, vetch, parsley and most Umbelliferae, 

 Leguminosae, Composite, and Labiatae are familiar examples : in 

 these, the pollen necessary for the fertilisation of any particular 

 flower usually comes from a younger one, because its own pollen 

 has been shed before the stigma is receptive. 



Protogynous flowers are less common : examples are seen in the 

 apple, pear, plantain {Plantago), meadow foxtail and sweet vernal- 

 grasses, rushes, hellebore, and species of Speedwell ( Veronica), 

 and Calceolaria. In these the stigmas are pollinated from the 



