FURTHER CHANGES IN THE SAP. 197 



out just now within the pickle jar. If they had been 

 left a little longer the rest would have died also. 

 What a wonder that with so many people in the 

 world, the air does not all get spoilt, like it did in the 

 Black Hole of Calcutta. 



But that bad kind of gas which is always being 

 produced by men and animals, and in some other 

 ways also, as you have seen by the burning candle, is 

 just what the plants want. Under the influence of 

 sun light they take it in, use up and live upon a part of it, 

 and give back another part of it to the atmosphere 

 again to make it pure and ready for our use. For 

 this carbonic acid gas,* so deadly to us, which the 

 plant takes in from the atmosphere, comes in contact 

 with those little green bodies which I told you are 

 found in certain cells, which are called chlorophyll 

 corpuscles (p. 184). Then, but only under the influence 

 of light, the gas and the corpuscle both become 

 changed. The gas is broken up ; part of it (oxygen 

 gas) which makes the atmosphere pure and good for 

 us to breathe again, is sent back again to do that 

 useful work. Part of it (carbon) joins with the 

 substance of the corpuscle, and so, as a general rule, 

 forms little grains of starch. This work, or process, 

 is called assimilation^ because the plant takes in 

 what it wants from the atmosphere (carbon), and 

 makes it part of itself, for the starch grains are the 



* cf. Appendix, "atmosphere," " carbonic acid gas." 



t From the Latin " assimilo" I make like. The act of converting 

 something into its own substance, cf. carbon in the Appendix. 



