48 Mendel's Experiments 



in the open as regards possible disturbance by insects. 

 Among the insects* which visit Peas the beetle Bruchus 

 pisl 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 

 uncovered!. 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, 

 however, a very slight one with Pisum, and is quite 

 incapable of disturbing the general result. Among more 

 than 10,000 plants which were carefully examined there 

 were only a very few cases where an indubitable false 

 impregnation had occurred. Since in the greenhouse such 

 a case was never remarked, it may well be supposed that 

 B ruckus pisi, and possibly also the described abnormalities 

 in the floral structure, were to blame. 



* [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.] 



