282 FERTILISATION IN FLOWERING PLANTS. 



In flowering plants various provisions are made for insuring the 

 application of the pollen to the stigma. The saccharine secretions of 

 the flower, the comparative length of the stamens and pistiLs, their 

 position, and the dehiscence of the anthers, are all regulated with this 

 view. The existence of spiral cells in the endothecium has reference 

 apparently to the bursting of the anther and the scattering of the 

 poUen. The number of pollen-grains produced is also very great. In 

 a floret of wheat Wilson reckoned about 7000 pollen-grains. Hassall 

 says that a single head of Dandelion produces upwards of 240,000, 

 each stamen of a Pseony 21,000, a Bulrush 144 grains by weight. 

 It has been stated that a single plant of Wistaria sinensis produced 

 5,750,000 stamens, and these, if perfect, would have contained 

 27,000,000,000 pollen-grains.* In a single flower of Maxillaria F. 

 Miiller estimated the poUen-grains at 34,000,000. This same flower 

 produces 1,756,000 seeds. In Orchis mascula the poUen-grains in a 

 single flower have been estimated at 120,000. In the case of Ever- 

 greens, such as Firs, the quantity of pollen is enormous, apparently 

 to insure its application notwithstanding the presence of leaves. The 

 poUen from pine forests has been wafted by the winds to a great 

 distance, and sometimes falls on the ground like a shower of sulphur. 

 It is thus that some kinds of coloured rain, occasionally witnessed, 

 may be accounted for. The pollen powder transmitted to considerable 

 distances remains floating in the air till carried down by a passing 

 shower. 



The quantity of pollen required for impregnation varies. Koel- 

 reuter says, that from fifty to si:^y grains of the pollen of Hibiscus 

 Trionum are required to fecundate the fruit completely, containing 

 about thirty ovules. The ovary of Mcotiana, Datura, Lychnis, and 

 Dianthus, according to Gsertner, may be completely fertilised by the 

 pollen of a single perfect anther. In Geum, from eight to ten anthers, 

 out of eighty-four to ninety-six contained in each flower, are sufficient 

 to fertilise from eighty to one hundred and thirty ovules contained in 

 the ovaries. 



In many trees in which the organs of reproduction are in separate 

 flowers (as in Hazel and Willow), the leaves are not produced until 

 fertilisation has been effected. The protection of the pollen from the 

 direct influence of moisture is efiected by the closing of the flowers, 

 by the elasticity of the anther-coat only coming into play in dry 



* The following estimate was made of the amount of flowers, stamens, etc., in a single 

 specimen of Wistaria sinensis ; — 



Number of clusters of Flowers 9,000 



individual Flowers 676,000 



Petals 3,375,000 



Stamens 6,750,000 



Ovules 4,050,000 



For the purpose of fertUising these ovules, the anthers, it perfect, would have contained 



about 27,000,000,000 poUen-grains, or about 7000 grains to each ovule. 



