OCR Text |
Show lo2 PISUM SATIVUM. CHAP, v. . at which the pollen-tubes are exserted; uamely, the early age ded were examined, and in seven of eight flowers not fully expan. thi's state. but they had not as th Hen-tubes were In ' . . . these e P0 . Although so few Insects VISit tho yet penetrated the .sti~:.a. country or in North Germany, and flowers of the pea Ill IS here to open abnormally soon, it does although the anthers see~ ·n its native country would be thus uot follow that the specws I circumstanced. . t' 1-avi·ng been self-fertilised for many 0 · to the vane 1es u . wm? d t their having been subjected In each genera-o ·eneratwns, an ° 'll b l . d . a to>. . the same conditions (as WI e exp ame m ~wn to nearly) I d'd ot expect that a cross between two such future chapter , I n · d t .· 1 I fit th ffspring. and so It prove on rm. n plants would ~ene e a~ plants o'f the Early Emperor pea, which 1867 I covere up sever 'ety so that it must already have w s not then a very new van ' . a . t d by self-fertilisation for at least a dozen genera-been propaga e rossed with pollen from a distinct t . Some flowers were c pwlannst. g·r owm. g I.l l the same row, a nd others weref allodw edth to fertilise themselves under a net. The two lots o see s us obtained were sown on opposite sides of two large potls, but~ onthl~ four pairs came up at the same tI.m e. The pots were cep6 m d 70 O'I'eenhouse. The seedlings of both lots when betwe~n an oin ches I. n hei. ght wer.e equa1 . When nearly full-grown they were measured, as in the following table :- No. of Pot. I. II. Total in inches. TABLE LVII. .Pisum suti'vum. Crossed Plants. Inches. 35 31g 35 37 138·50 Self-fertilised Plants. Inches. 29* - - 51 45 33 - - 158•76 The average height of the four crossed P1 ~ n t s.i s here 34·62, 100 to 115. and that of the four self-fertilised plants 3~. 68, or ::If-fertilised, So that the crossed plants, far from beatmg the were completely beaten by them. CHAP. v. SAROTHAMNUS SCOPARIUS. 163 There can be no doubt that the result would have been widely different, if any two varieties out of the numberless ones which exist had been crossed. Notwithstanding that both had been self-fertilised for many previous generations, each would almost certainly have possessed its own peculiar constitution; and this degree of differentiation would have been sufficient to make a cross highly beneficial. I have spoken thus confidently of the benefit which would have been derived from crossing any two varieties of the pea from the following facts : Andrew Knight in speaking of the results of crossing reciprocally very tall and short varieties, says,* "I had in this experiment a striking instance of the stimulative effects of crossing the breeds; for the smallest variety, whose height rarely exceeded 2 feet, was increased to 6 feet; whilst the height of the large and luxuriant kind was very little diminished." Recently Mr. Laxton has made numerous crosses, and everyone has been astonished at the vigour and luxuriance of the new varieties which he has thus raised and afterwards fixed by selection. He gave me seed-peas produced from crosses between four distinct kinds; and the plants thus raised were extraordinarily vigorous, being in each case from 1 to 2 or even 0 feet taller than the parent-forms, which were raised at the same time close alongside. But as I did not measure their actual height I cannot give the exact ratio, but it must have been at least as 100 to 75. A similar trial was subsequently made with two other peas from a different cross, and the result was nearly the same. For instance, a crossed seedling between the Maple and Purple-podded pea was planted in poor soil and grew to the extraordinary height of 116 inches; whereas the tallest plant of either parent variety, namely, a Purple-podded pea, was only 70 inches in height; or as 100 to 60. SAROTHAMNUS SCOPARIUS. Bees incessantly visit the flowers of the common Broom, and these are adapted by a curious mechanism for cross-fertilisation. When a bee alights on the wing-petals of a young flower, the keel is slightly opened and the short stamens spring out, which rub their pollen against the abdomen of the bee. If a i·ather older flower is visited for the first time (or if tho bee exerts great force on a younger flower), the keel opens along its whole length, and the longer as well as the shorter stamens, together * 'Philosophical Tr::msactions,' 1799, p. 200. lVI 2 |