4io History of the Sexual Theory. [BOOK in. 



Again we find observations on the movements of anthers 

 and stigmas, especially those caused by sensitiveness. Count 

 Giambattista dal Covolo had made the first observations in 

 1764 on the sensitiveness of the anthers of thistle-like plants, 

 and had endeavoured to explain their mechanism. Koelreuter 

 did not trouble himself about this point, so much as about 

 the connection between the irritability of the anthers and the 

 pollination of the stigmas. He took into consideration the 

 sensitive stamens of Opuntia, Berberis and Cistus, which Du 

 Hamel had already noticed, and discovered for himself the 

 irritability of the lobes of the stigma in Martynia proboscidea 

 and Bignonia radicans. He noticed that the lobes when 

 touched close, but soon open again ; but if pollen is placed 

 upon them, they remain closed till fertilisation is secured. 



How perfectly insects effect the pollination of flowers he 

 showed by a comparative trial, in which he applied pollen 

 himself to three hundred and ten flowers with a brush, while 

 he left the same number to the operation of insects ; the 

 number of seeds formed in the latter case was very little less 

 than in the former, though the insects had to contend with 

 unfavourable weather. 



He endeavoured also to ascertain the time required for the 

 quantity of ' seminal matter ' sufficient for fertilisation to reach 

 the ovary after pollination ; he also showed that pollination 

 is followed by fertilisation without the aid of light ; later 

 botanists incorrectly maintained the contrary. 



Koelreuter was less successful in his observations on the 

 structure of pollen-grains ; here the microscope was indis- 

 pensable and microscopes were still very imperfect. Never- 

 theless he discovered that the outer covering of the pollen-grain 

 consists of two distinct coats, and noticed the spines and 

 sculpturings on the outer coat and its elasticity ; he observed 

 the lids of the orifices in the exine of Passiflora coerulea, and 

 went so far as to see the inner coat in moistened pollen-grains 

 protrude in the form of conical projections, which then however 



