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Show 394 MEANS OF CROSS-FERTILISATION. CHAP. X. The onion produces a large number of flowers, all crowded together into a large globular head, each flower having six stamens; so that the stig1nas receive plenty of pollen from their own and the adjoiuinO' ~nthers. Consequently the plant is fairly self-fertile when protected from insects. A blood-red, silver, globe and Spanish onion were planted near together; and seedlings were raised fro1n each kind in four separate beds. In all the beds mongrels of various kinds were numerous, except a1nongst the ten seedlings from the blood-red onion, which included only two. Altogether forty-six seedlings were raised, of which thirty-one had been plainly crossed. A similar result is known to follow with the varieties of many other plants, if allowed to flower near together: I refer here only to species which are capable of fertilising themselves, for if this be not the case, they would of course be liable to be crossed by any other variety growing near. Horticulturists do not commonly distinguish between the effects of variability ancl intercrossing; but I have collected evidence .on the natural crossing of varieties of the tulip, hyacinth, anemone, ranunculus, strawberry, Leptos£phon androsaceus, orange, rhododendron and rhubarb, all of which plants I believe to be self-fertile.* Much other indirect evidence could ron, 'De l'Ei:jpece,' tom. ii. p. 50, mal\es an analogous statement with respect to thi8 plant. * With respect to tulips and some other flowers, sec Godron, 'Del' Espece,' tom. i. p. 252. For am·mones, ' Gard. Chron.' 1859, p. 98. For strawberries, see H erbert in 'Transact. of Hort. Soc.' vol. iv. p. 17. The same observer elsewhere speaks of the spontaneous crossing of rhododendrons. Gallesio makes the same state- · ment with respeet to oranges. I have myself known extensive crossing to oceur with the comnlon rhubarb. For l ,eptosiphon, Verlot, ' Des Vnrietcs,' 18G5, p. 20. I have not included in my list tlle C~rnation, Nom(lphila, or ~nti~·~ rl1inulll tho varieties of winch 918 known 'to cross freely, bccausfe these plants are not alwAys selfol'tilo. I know nothin~ about the self-fertility of Tr?lh~!S (Lccoq, 'De ]a Fccondntlo~, 186.2, p 93\ Mahonia and Cnnum, l!l w.hic{; genera th~ species intercross CHAP. X. PREPOTENT POLLEN. 395 be given with respect to the extent to which varieties of the same species spontaneously intercross. Gardeners who rais.e seed for sale are compelled by dearly bought expenence to take extraordinary precautions against int~rcrossing. Thus Messrs. Sharp "have land engaged In the growth of seed in no less than eight parishes." The mere fact of a vast number of plants. belongi~g to the same variety growing together Is a considerable protection, as the chances are strong in favour of plants of the same. variety interCl'ossing; and it is in chief part owing to this circumstance, that certain villages have become fainous for pure seed of particular varieties.* Only two trials were made by me to ascertain after how long an interval of time, pollen from a distinct variety would obliterate more or less completely the action of a plant's own pollen. The stigmas in two lately expanded flowers on a variety of cabbage, called Ragged Jack, were well covered with pollen from the same plant. After an interval of twenty-three hours, pollen from the Early Barnes Cabbage growing at a distance was placed on both stigmas; and as the plant was left uncovered, pollen from other flowers on the Ragged Jack would certainly have been left by the bees during the next two or three days on the same two stigmas. Under these circumstances it seemed very unlikely that the pollen of the Barnes cabbage would produce any effect; hut three out of the fifteen plants raised from the two capsules thus produced were plainly mongrelised: and I have no doubt that the twelve other plants were ~a~gely. With respect to 1\'Iahonia, so fl'eely that pure seed could not Jt 1 B ~ow s~arcely possible to pro- be saved. cure m th1s country pure speci- * With re pect to Messrs. Sharp, mens of Jll .. aquijolium or repens; see 'Gardenf'rs' Chronicle,' 1856, and the vannus spf'cies of Cl'imun p. 823. Lindley's' Theory of Hor-sent? by Herbert(' Amaryllidacero,' ticulture,' p. 319. p. 3-) to Calcutta, crosserl there |
