in Hybridisation 341 



From a larger number of plants of the same variety only 

 the most vigorous were chosen for fertilisation. Weakly 

 plants always afford uncertain results, because even in the 

 first generation of hybrids, and still more so in the sub- 

 sequent ones, many of the offspring either entirely fail to 

 flower or only form a few and inferior seeds. 



Furthermore, in all the experiments reciprocal crossings 

 were effected in such a way that each of the two varieties 

 which in one set of fertilisations served as seed-bearer in 

 the other set was used as the pollen plant. 



The plants were grown in garden beds, a few also 

 in pots, and were maintained in their naturally upright 

 position by means of sticks, branches of trees, and strings 

 stretched between. For each experiment a number of pot 

 plants were placed during the blooming period in a green- 

 house, to serve as control plants for the main experiment 

 in the open as regards possible disturbance by insects. 

 Among the insects^ which visit Peas the beetle Bruchus 

 pisi might be detrimental to the experiments should it 

 appear in numbers. The female of this species is known 

 to lay the eggs in the flower, and in so doing opens the 

 keel ; upon the tarsi of one specimen, which was caught in 

 a flower, some pollen grains could clearly be seen under a 

 lens. Mention must also be made of a circumstance which 

 possibly might lead to the introduction of foreign pollen. 

 It occurs, for instance, in some rare cases that certain parts 

 of an otherwise quite normally developed flower wither, 

 resulting in a partial exposure of the fertilising organs. A 

 defective development of the keel has also been observed, 

 owing to which the stigma and anthers remained partially 

 uncoveredf. It also sometimes happens that the pollen 

 does not reach full perfection. In this event there occurs 

 a gradual lengthening of the pistil during the blooming 

 period, until the stigmatic tip protrudes at the point of the 

 keel. This remarkable appearance has also been observed 

 in hybrids of Phaseolus and Lathyrus. 



The risk of false impregnation by foreign pollen is, 



* [It is somewhat surprising that no mention is made of Thrips, which 

 swarm in Pea flowers. I had come to the conclusion that this is a real 

 source of error and I see Laxton held the same opinion.] 



t [This also happens in Sweet Peas.] 



