CONDITIONS OF PLANT LIFE 121 



Sulphur, which is an important constituent of albumen, is derived 

 from the sulphates of the soil. In addition to the above, there are 

 other elements, sometimes described as non-essential constituents of 

 plants. Amongst these are silicon (to give stiffness), sodium, chlorine, 

 iodine, Iromine, etc. All these elements contribute to the formation 

 or quality of the protoplasm of plants. 



The gases essential to plants, and absorbed as such, are two : 

 Carbon dioxide (carbonic acid) and Oxygen ; the necessary hydrogen 

 and nitrogen being absorbed in the form of salts. By the aid of the 

 green chlorophyll corpuscles, and under the influence of sunlight, we 

 know that leaves absorb the carbon dioxide of the atmosphere, and 

 effect certain changes in it. The hydrogen, as we have seen, is 

 obtained from the water. Oxygen is absorbed through the leaves and 

 through the root from the interstices of the soil. Each of these gases 

 contributes vitally to the existence of the plant. The fourth gas, nitro- 

 gen, which constitutes more than two-thirds of the air we breathe (79 

 per cent, of the total volume and 77 per cent, of the total weight of the 

 atmosphere), is also an absolutely necessary food required by plants. 

 Yet, although this is so, the plant cannot absorb or obtain its nitrogen 

 in the same manner in which it acquires its carbon — viz., by absorption 

 through the leaves — nor can the plant take nitrogen into its own 

 substance by any means as nitrogen. Hence, although this gas is 

 present in the atmosphere surrounding the plant, the plant will 

 perish if nitrogen does not exist in some combined form in the soil. 

 Nitrates and compounds of ammonia are widely distributed in nature, 

 and it is from those bodies that the plant obtains, by means of its 

 roots, the necessary nitrogen. 



Until comparatively recently it was held that plant life could not 

 be maintained in a soil devoid of nitrogen or compounds thereof. 

 But it has been found that certain classes of plants (the Leguminosce 

 for example), when they are grown in a soil which is practically free 

 from nitrogen at the commencement, do take up this gas iuto their 

 tissues. One explanation of this fact is that free nitrogen becomes 

 converted iato nitrogen compounds in the soil through the influence 

 of micro-organisms present there. Another explanation attributes 

 this fixation of free nitrogen to micro-organisms existing in the 

 rootlets of the plant. These two classes of organisms, known as the 

 nitrogen-fixing organisms, will require our consideration at a later 

 stage. Here we merely desire to make it clear that the main supply 

 of this gas, absolutely necessary to the existence of vegetable life 

 upon the earth, is drawn not from the nitrogen of the atmosphere, 

 but from that contained in nitrogen compounds in the soil. The 

 most important of these are the nitrates. Here then we have the 

 necessary food of plants expressed in a sentence : water, inorganic salts, 

 gases ; some of the salts containing nitrogen in the form of nitrates. 



