WATER-PLANTS AS SCAVENGERS. 171 



however, the quantity is so small that, as has been said, 

 " one might safely pass the night in a greenhouse without 

 any danger of being suffocated by the geraniums." Such 

 plants as the colza, pea, bean, raspberry, and sunflower, 

 exhale during a whole night only as much as they absorb 

 during one quarter of an hour or twenty minutes of direct 

 sunshine. 



Seaweeds, as well as land plants, perform the office 

 of scavengers, not only by absorbing carbonic acid, but 

 by giving out oxygen ; and Dr. Hooker, remarking upon 

 the universality of the diatoms, speaks of them as a most 

 important feature of the Polar seas, where the higher forms 

 of vegetation are so scarce that the office of purifying the 

 waters devolves mainly upon them. 



Many fresh-water plants, among which the common 

 duckweed is prominent, are powerful purifiers, whether 

 their leaves float below or upon the surface ; and those 

 who have kept aquariums will, no doubt, have noticed 

 the little globules of oxygen which cover the weeds when 

 the sun shines upon them. By the oxygen thus evolved 

 the impurities in the water are literally burnt up.* 



The carbon taken up by plants is in some unknown 

 manner combined by them with oxygen and hydrogen to 

 make cellulose, the colourless material of which all young 

 vegetable fibres are composed. It is to be seen in the 

 skeleton leaves, sometimes picked up in winter, with all 



* Mere exposure to the air will purify water to a great extent. Ozone, 

 or condensed oxygen, generated by electricity, is a yet more powerful con- 

 sumer of all putrescible matter in the air. The air of impure places is 

 universally characterised by a want of oxygen, and the differences, though 



