22 INTRODUCTORY REMARKS. CHAP. I, 



therefore not exposed during successive years to such 

 great vicissitudes of climate as are plants growing out 

 of doors. 



On some apparent and real Causes of Error in my Ex- 

 periments. It has been objected to such experiments 

 as mine, that covering plants with a net, although only 

 for a short time whilst in flower, may affect their health 

 and fertility. I have seen no such effect except in one 

 instance with a Myosotis, and the covering may not 

 then have been the real cause of injury. But even if 

 the net were slightly injurious, and certainly it was not 

 so in any high degree, as I could judge by the appear- 

 ance of the plants and by comparing their fertility with 

 that of neighbouring uncovered plants, it would not 

 have vitiated my experiments ; for in all the more im- 

 portant cases the flowers were crossed as well as self- 

 fertilised under a net, so that they were treated in this 

 respect exactly alike. 



As it is impossible to exclude such minute pollen- 

 carrying insects as Thrips, flowers which it was intended 

 to fertilise with their own pollen may sometimes have 

 been afterwards crossed with pollen brought by these 

 insects from another flower on the same plant ; but as 

 we shall hereafter see, a cross of this kind does not 

 produce any effect, or at most only a slight one. When 

 two or more plants were placed near one another 

 under the same net, as was often done, there is some 

 real though not great danger of the flowers which 

 were believed to be self-fertilised being afterwards 

 crossed with pollen brought by Thrips from a distinct 

 plant. I have said that the danger is not great, 

 because I have often found that plants which are 

 pelf-sterile, unless aided by insects, remained sterile 

 when several plants of the same species were placed 



