CHAP X. MEANS OP CROSS-FERTILISATION. 377 



phons the two most precious of all the elements for 

 the growth of plants but in the case of most open 

 flowers, a large quantity of pollen is consumed by 

 pollen-devouring insects, and a large quantity is 

 destroyed during long-continued rain. With many 

 plants this latter evil is guarded against, as far as 

 is possible, by the anthers opening only during dry 

 weather,* by the position and form of some or all of 

 the petals, by the presence of hairs, &c., and as Kerner 

 has shown in his interesting essay, f by the movements 

 of the petals or of the whole flower during cold and wet 

 weather. In order to compensate the loss of pollen in 

 so many ways, the anthers produce a far larger amount 

 than is necessary for the fertilisation of the same flower. 

 I know this from my own experiments on Ipomcea, 

 given in the Introduction ; and it is still more plainly 

 shown by the astonishingly small quantity produced 

 by cleistogamic flowers, which lose none of their pollen, 

 in comparison with that produced by the open flowers 

 borne by the same plants ; and yet this small quantity 

 suffices for the fertilisation of all their numerous seeds. 

 Mr. Hassall took pains in estimating the number of 

 pollen-grains produced by a flower of the Dandelion 

 (Leontodon), and found the number to be 243,600, 

 and in a Paeony 3,654,000.f A single plant of Typha 

 produced 144 grains by weight of pollen, and as this 

 plant is anemophilous with very small pollen-grains, 



* Mr. Blackley observed that up in the atmosphere, during the 

 the ripe anthers of rye did not first fine and dry days after wet 

 dehisce whilst kept under a bell- weather, than at other times : 

 glass in a damp atmosphere, ' Experimental Researches on 

 whilst other anthers exposed to Hay Fever,' 1873, p. 127. 

 the same temperature in the open t ' Die Schutzmittel des Pol- 

 air dehisced freely. He also found lens,' 1873. 

 much more pollen adhering to J 'Annals and Mag. of Nat 

 the sticky slides, which were Hist.' vol. yiii. 1842, p. 108. 

 attache! to kites and sent high 



