72 BOTANY. [chap. ii. 



from the atmosphere and oxygen restored to it during 

 the presence of light, in the act of nutrition than is the 

 converse in the function of respiration even during the 

 whole dajj consequently the statement that plants purify 

 the atmosphere is perfectly true, yet it is necessary to 

 remember clearly the manner in which this function is 

 effected. 



If the above explanation has been understood, it will 

 be seen that living plants tend to purify the atmosphere 

 of an apartment during the day by removing much more 

 carbonic dioxide from the atmosphere than they give off, 

 and also by adding oxygen to the air, whereas during 

 the night oxygen only is removed from the atmosphere 

 and carbonic acid alone liberated, thus rendering the 

 atmosphere impure for animal life. The above are the 

 results that would be obtained by experimenting with 

 plants in a closed volume of air under the two given 

 conditions, but in a well-ventilated room, unless the 

 amount of plant life is excessive, on account of the 

 slow process of respiration, no inconvenience would be 

 experienced. 



The element nitrogen, being one of the constituents of 

 protoplasm, is required by all plants, and, although 

 abundant in the atmosphere, is never taken in direct in 

 this form, but is obtained from salts of ammonia and 

 nitrates present in the soil. Some plants, however, do 

 not obtain the whole of their nitrogen in this manner, 

 but, according to the extent of differentiation they have 

 undergone in connection with this function, obtain a 

 greater or less amount from members of the Animal 

 Kingdom, and for this reason are known as carnivorous 

 'plants, or the term insectivorous . plants is sometimes 



