Chap. I. INTRODUCTORY REMARKS. 11 



the flowers touch the net they may be cross-fertilised 

 by bees, as I have known to happen ; and when the net 

 is wet the pollen may be injured. I used at first 

 " white cotton net," with very fine meshes, but after- 

 wards a kind of net with meshes one-tenth of an 

 inch in diameter ; and this I found by experience 

 effectually excluded all insects excepting Thrips, which 

 no net will exclude. On the plants thus protected 

 several flowers were marked, and were fertilised with 

 their own pollen ; and an equal number on the same 

 plants, marked in a different manner, were at the same 

 time crossed with pollen from a distinct plant. The 

 crossed flowers were never castrated, in order to make 

 the experiments as like as possible to what occurs 

 under nature with plants fertilised by the aid of 

 insects. Therefore, some of the flowers which were 

 crossed may have failed to be thus fertilised, and 

 afterwards have been self-fertilised. But this and 

 some other sources of error will presently be discussed. 

 In some few cases of spontaneously self-fertile species, 

 the flowers were allowed to fertilise themselves under 

 the net ; and in still fewer cases uncovered plants were 

 allowed to be freely crossed by the insects which in- 

 cessantly visited them. There are some great advan- 

 tages and some disadvantages in my having occasion- 

 ally varied my method of proceeding ; but when there 

 was any difference in the treatment, it is always so 

 stated under the head of each species. 



Care was taken that the seeds were thoroughly 

 ripened before being gathered. Afterwards the crossed 

 and self-fertilised seeds were in most cases placed on 

 damp sand on opposite sides of a glass tumbler covered 

 by a glass plate, with a partition between the two lots ; 

 and the glass was placed on the chimney-piece in a 

 warm room. I could thus observe the germination of 



