1964 



A TEXTBOOK OF THEORETICAL BOTANY 



irritability of the anther filaments in this case is a contrast to the move- ! 

 ment of the style already described in the case of Arctotis. Investigations 

 by Juel have shown that irritability of the stamens occurs in a wide ; 

 range of species included in all the tribes of the family. In a few cases, 

 e.g., Gerbera, the pollen is extruded with almost explosive rapidity. Irrita- 

 bility of the style however appears to be restricted to the single tribe 

 Arctotideae. 



Our last example of pollination mechanisms is one from the Cichorieae, 

 and we may use Hierachim umbeUatum (Fig. 1893) as a type. The florets are 

 all yellow and the part of the style projecting from the anther cylinder is 

 completely covered with spinose sweeping hairs, while the inner surface 

 of the stylar branches is beset with stigmatic papillae. If the pollen is 



B 



Fig. 1893. — Hieracium umbeUatum. A, Floret with exserted 

 style showing the spiny zone below the stigmas, which 

 sweeps pollen from the anther tube. B, Stigmas re- 

 curved so that they touch the pollen-bearing zone and 

 get self-pollinated. (After Kmith.) 



removed from the hairs as a result of insect visits then the floret can only 

 be cross-pollinated. If on the other hand the pollen is not removed, self- 

 pollination is effected by the bending back of the stigmatic branches until 

 the papillae come into contact with the pollen on the sweeping hairs. 

 Species of Hieracium are visited both by flies and by bees. Occasionally 

 butterflies may also visit them to collect nectar. 



In the allied genus Taraxacum experiments have shown that if both 

 styles and anthers are removed from hermaphrodite florets they are able to 

 produce viable seeds. The only conclusion which can be drawn, therefore, is 

 that in such species the seeds develop parthenogenetically, and it is to this 

 parthenogenetic propagation of chance hybrids between sub-species that 

 the great multiplicity of apparent species in the genus is due. 



