238 THE LIFE OF THE PLANT 



buds, guards the inner more delicate organs of the 

 flower during their development ; the latter plays the 

 same part towards the ovules enclosed within it. In 

 the experiments on the fertilisation of ovules detached 

 from the ovary, that we have just described, the 

 experimenter met considerable difficulties in struggling 

 with minute parasitic organisms, bacteria, which have 

 so terrible a reputation owing to the infectious diseases 

 they produce. When we wish to preserve organic 

 matter from decay, we must protect it from becoming 

 infected with bacteria. This we do by keeping it in 

 hermetically closed vessels, or at least guarding it from 

 the access of the germs of these organisms, floating 

 about in the air, by closing the necks of the vessels with 

 cotton wool. The cavity of the ovary is exactly the 

 kind of vessel in which the ovule and the seed may 

 safely develop without coming into contact with the 

 germs of parasitic bacteria floating in the air. But 

 an objection may be raised here : if on reaching 

 the surface of the stigma the pollen grains germinate 

 and their pollen-tubes reach the ovule, why should 

 not bacteria flying about in the air likewise develop 

 on the stigma and reach the ovule in the same way ? 

 The same observer to whom we are indebted for the 

 explanation just brought forward of the significance of 

 the ovary, has also explained the special significance 

 of the stigma. Pollen grains developing outside the 

 flower (under the microscope) suffer from bacteria in 

 the same way as ovules. In order to prevent their 

 attacks he slightly acidified the liquid in which the 

 pollen grains germinated. This slightly acid reaction, 

 which was harmless to the pollen grains, did actually 

 prevent the development of bacteria. On testing the 

 stigma itself, he found it also had an acid reaction. 

 Thus, while it lets the pollen-tubes pass through, the 

 stigma apparently prevents the access of bacteria to the 

 ovary. 



