28o GLIMPSES IX TO PL A XT-LIFE 



we must remember that the gases necessar}' for 

 phmt food form j)art of the air we breathe; this 

 air is made up of two-thirds nitrogen, one-third 

 ox}'gen, witli a small and \ar}-ing, but always 

 present, quantit}' of carbon-dioxide, and of these 

 the latter is the most essential to the life of 

 plants. 



We have learnt in our stud)- of the leaf how it, 

 b\- the aid of the green chlorophj'll granules, and 

 under the influence of sunlight, absorbs this carbon 

 dioxide and effects certain changes in it. One of 

 the most essential elements in the growth of plants 

 is nitrflo;cn ; this we have just seen constitutes two- 

 thirds (jf the air we breathe, but the plant is unable 

 to make use of it in this free form ; that is to sa\', 

 although the leaf can freely absorb carbon-dioxide 

 it cannot absorb nitrogen ; it has to be taken in 

 by the roots of ordinary plants in the form of 

 nitrates, that is, in conjunction with some other 

 element. There is, however, an important ex- 

 ception to this rule ; for what are called the 

 insectivorous plants have the power to absorb 

 nitrogen under certain conditions. These will be 

 explained in the succeeding chapter. We can 



