154 STERILITY. Chap. XVIII. 



ascend mountains beyond the height at which they can produce 

 seed. 110 Certain species of Poa and Festuca, when growing on 

 mountain-pastures, propagate themselves, as I hear from Mr. 

 Bentham, almost exclusively by bulblets. Kalm gives a more curious 

 instance 111 of several American trees, which grow so plentifully in 

 marshes or in thick woods, that they are certainly well adapted for 

 these stations, yet scarcely ever produce seeds ; but when acciden- 

 tally growing on the outside of the marsh or wood, are loaded with 

 seed. The common ivy is found in Northern Sweden and Eussia, 

 but flowers and fruits only in the southern provinces. The Acorus 

 calamus extends over a large portion of the globe, but so rarely 

 perfects fruit that this has been seen only by a few botanists; 

 according to Caspary, all its pollen-grains are in a worthless condi- 

 tion. 112 The Hypericum catycinum, which propagates itself so freely 

 in our shrubberies by rhizomes, and is naturalised in Ireland, 

 blossoms profusely, but rarely sets any seed, and this only during 

 certain years ; nor did it set any when fertilised in my garden by 

 pollen from plants growing at a distance. The Lydmachia nummu- 

 laria, which is furnished with long runners, so seldom produces 

 seed-capsules, that Prof. Decaisne, 113 who has especially attended to 

 this plant, has never seen it in fruit. The Carex rigida often fails 

 to perfect its seed in Scotland, Lapland, Greenland, Germany, and 

 New Hampshire in the United States. 114 The periwinkle ( Vinca 

 minor), which spreads largely by runners, is said scarcely ever to 

 produce fruit in England ; 115 but this plant requires insect-aicl for 

 its fertilisation, and the proper insects may be absent or rare. The 

 Jussicea grand i [flora has become naturalised in Southern France, and 

 has spread by its rhizomes so extensively as to impede the naviga- 

 tion of the waters, but never produces fertile seed. 116 The horse- 

 radish (Cockleuria armoracia) spreads pertinaciously and is natural- 

 ised in various parts of Europe ; though it bears flowers, these 

 rarely produce capsules : Professor Caspary informs me that he has 

 watched this plant since 1851, but has never seen its fruit; 65 per 

 cent, of its pollen-grains are bad. The common Ranunculus ficaria 

 rarely bears seed in England, France, or Switzerland ; but in 1863 

 I observed seeds ou several plants growing near my house. 117 Other 



110 Wahlenberg specifies eight Zool., torn. iv. p. 280. Prof. Decaisne 

 species in this state on the Lapland refers also to analogous cases with 

 Alps: see Appendix to Linna?us''Tour mosses and lichens near Paris. 



in Lapland,' translated by Sir J. E. " 4 Mr. Tuckermann, in Silliman's 



Smith, vol. ii. pp. 274—280. 'American Journal of Science,' vol. 



111 'Travels in North America,' xlv. p. 1. 



Eng. translat., vol. iii. p. 175. lls Sir J. E. Smith, ' English Flora,' 



112 With respect to the ivy and vol. i. p. 339. 



Acorus, see Dr. Brom field in the ' Phy- m G. Planchon, 'Flora de Mont- 



tologist,' vol. iii. p. 376. Also Lind- pellier,' 1864, p. 20. 

 ley and Vaucher on the Acorus, and " 7 On the non-production of seeds 



see Caspary as below. in England, see Mr. Crocker, in ' Gar- 



113 'Aunal. des Sc. Nat.,' 3rd series, dener's Weekly Magazine,' 1852, p. 



