378 MEANS OP CROSS-FEETTLISATION. Chap. X, 



their number in the above weight must have been 

 prodigious. We may judge of this from the following 

 facts : Dr. Blackley ascertained * by an ingenious 

 method, that in the three following anemophilous plants, 

 a single grain-weight of the pollen of Lolium perenne 

 contained 6,032,000 grains ; the same weight of the 

 pollen of Plantago lanceolata contained 10,124,000 

 grains ; and that of Scirpus lacustris, 27,302,050 grains. 

 Again Mr. A. S. Wilson estimated by micro-measure- 

 mentf that a single floret of rye yielded 60,000 pollen- 

 grains, whilst one of spring wheat yielded only 6864 

 grains. The editor of the ' Botanical Eegister ' counted 

 the ovules in the flowers of Wistaria sinensis, and care- 

 fully estimated the number of pollen-grains, and he 

 found thatforeach ovule there were 7000 grains.^ With 

 Mirabilis, three or four of the very large pollen-grains 

 are sufficient to fertilise an ovule ; but I do not know 

 how many grains a flower produces. With Hibiscus, 

 Kolreuter found that sixty grains were necessary to fer- 

 tilise all the ovules of a flower, and he calculated that 

 4863 grains were produced by a single flower, or eighty- 

 one times too many. With Geum urlanum, however, 

 according to Gartner, the pollen is only ten times too 

 much. As we thus see that the open state of all 

 ordinary flowers, and the consequent loss of much 

 pollen, necessitate the development of so prodigious an 

 excess of this precious substance, why, it may be asked, 

 are flowers always left open ? As many plants exist 

 throughout the vegetable kingdom which bear cleisto- 

 gamic flowers, there can hardly be a doubt that all 



* 'New Observations on Hay 1846, p. 771. 

 Fever,' 1877, p. 14. Kolreuter, * Vorlaufige Nach- 



t ' Gardeners' Chronicle,' March richt,' 1761, p. 9. Gartner, ' Bei 



1874, p. 376. trage zur Kenntniss,' &c. p. 346. 



X Quoted in ' Gard. Chron.' 



