432 REPRODUCTION. 



Those of Hibiscus Trionum at least three days after removal 

 from the anther ; those of Chcirauthus Cheiri, fourteen days ; 

 those of Camellia, Cannabis, Zea, and Phrenix dactylifera (Date), 

 one year. 



1122. Although each ovule requires for its impregnation only 

 one pollen-tube, the number of pollen-grains in flowers which 

 open at maturity is far in excess of the number of ovules. The 

 ratio has been ascertained in a few cases, among which are 

 the following : Cereus grandiflorus, 1 250,000 grains of pollen 

 to 30,000 ovules ; Wistaria sinensis, 2 about 7,000 grains of 

 pollen to each ovule ; Hibiscus Trionum, 8 4,863 grains of pol- 

 len to about 30 ovules. In some other cases, for instance Geum 

 urbanum, 4 the excess of pollen over ovules is about 10 : 1. 



1123. The localization of the conductive tissue in the ovary 

 itself is sometimes very marked ; thus in ovaries with parietal 

 placentation, the ovarian walls in the immediate vicinity of the 

 ovules are seen to be distinctly conductive, while in those with 

 axile placentation, the modified tissue is found in the axis. 

 Capus distinguishes the following varieties of conductive pla- 

 centae : (1) with a smooth surface, the micropyle being close 

 to the placenta, e. g., Solanum ; (2) papillar, the papillae either 

 simple or compound, sometimes serving to guide the pollen-tube 

 to the micropyle, e. g., some Cucurbitaceae ; (3) hairy, the hairs 

 sometimes secreting a mucus or even breaking down into a 

 gelatinous mass through which the pollen-tube may penetrate 

 with facility, e. g., some Aroids. Special names were formerly 

 given to peculiar forms of the conductive tissue, but the terms 

 now possess no utility. For special examples of the forms, the 

 reader must consult the practical exercises at the end of this 

 volume. 



1124. Structure of the ovule. As shown on page 175, the 

 ovules arise as minute protuberances at some part of the ova- 

 rian wall or upon the axis of the ovary. In orchids the pro- 

 tuberance consists of only a single row of cells ; but in most 



1 Morren. a Gardeners' Chronicle, 1846, p. 771. 



3 Kolreuter : Vorlaufige Nachricht (quoted by Balfour : Class Book of 

 Botany, p. 564). 



* Gartner : Beitrage zur Kenntniss, p. 346 (quoted by Darwin in "Effects 

 of Cross and Self Fertilization in the Vegetable Kingdom," p. 377). 



The following are some of Hassall's determinations of the number of pollen- 

 grains (Annals of Nat. Hist, viii., 1842, p. 108): Dandelion, 243,600 grains ; 

 a flower of Peony, with 174 stamens each containing 21,000 pollen-grains, 

 3,654,000 ; while in a plant of Rhododendron the number of grains was esti- 

 mated to be 72,620,000. 



