CHAP. V. SABOTHAMNUS SCOPARIUS. 163 



There can be no doubt that the result would have been 

 widely differe.it, 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." Eecently 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 8 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 acd 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. 



SAEOTHAMNUS SCOPARIUB. 



P>ees 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 rather 

 older flower is visited for the first time (or if the 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 Transactions,' 1799, p. 200. 



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