133 



LECTURE XI 

 THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 



As the result of changes taking place in the carbohydrates which originate 

 in the chloroplast, there arises a large number of important vegetable sub- 

 stances, of which only the materials of the cell-wall, fats, and the numerous 

 organic acids may be alluded to here. All these substances are composed of the 

 elements carbon, hydrogen, and oxygen only, but over and above there are 

 numerous compounds in the plant which contain a fourth element, nitrogen ; 

 every plant, in fact, contains this element in a small but constant percentage 

 (Lecture I, p. 5). The form in which nitrogen can be utilized varies in the 

 different types, but we will confine ourselves at present to nitrogen requirements 

 of the green plant oi whose absorption of nutriment we have obtained so limited 

 a conception. We certainly know far less about the mode of assimilation 

 of nitrogen than we do of carbon, and this is the more to be regretted as 

 nitrogen is an even more important food material than carbon. For protoplasm, 

 the actual living substance, always contains nitrogen, while on the contrary 

 those bodies which are composed of carbon, oxygen, and hydrogen only, 

 cannot be considered as endowed with vitality. 



Let us return to the water and sand-culture methods by means of which 

 we were enabled to arrive at such definite conclusions as to the requirements 

 of plants so far as the materials of the ash were concerned. When plants 

 were grown in nutritive solutions we found that a very considerable increase 

 in the plant's dry weight took place, but we also learned that such culture 

 fluids must contain all the materials needful to the support of plant hfe 

 (p. 81). The nitrogen supplied was in the form of a nitrate of calcium or 

 potassium. We have now to settle the question as to whether such an 

 addition is really necessary, whether the enormous quantity of free nitrogen, 

 amounting to four-fifths of the atmosphere, may not be utilized by the 

 plant. The answer to this question is emphatically in the negative ; for 

 although we may know of methods by which free nitrogen is brought into 

 combination in inorganic nature, and although we shall, later on, find that there 

 are certain plants (Lecture XIX) which are able to make use of free nitrogen, 

 still we are compelled to deny this power to the ordinary green plant. 



It is to BoussiNGAULT (1860-61) that we owe the establishment of this view; 

 he was certainly unaware of the special powers possessed by Leguminosae, 

 although he carried out not a few researches on these plants as well. Since we 

 purpose dealing with the problem of the absorption of nitrogen by the Legu- 

 minosae separately in Lecture XIX, we will confine ourselves here to plants 

 not belonging to that group, and take as our example Helianthus argophyllus. 

 BoussiNGAULT performed three series of experiments on this plant ; in the 

 first series he grew the plants in pure sand without any addition of minerals, 

 and especially with the omission of combined nitrogen ; in the second series 

 the sand had added to it the materials of the ash and potassium nitrate ; in 

 the third series materials of the ash and, in addition, potassium carbonate instead 

 of potassium nitrate. The result of the research is summarized in the following 

 table :— 



Dry substance ; Organic sub- Gain in carbon Gain in nitro- 



seedtaken as=i. stance formed. in 86 days. gen in 86 days. 



A. (Sand) ... 3.6 0-285 ^g). 0.114(g). 00023(g). 



B. (Sand, ash, nitrate) 1983 211 11 ,, 8-444 >> o-i666 „ 



C. (Sand, ash, carbonate) 46 0391 ,, 0-156 „ 00027 ,, 



We see from these numbers that in Series A and C nitrogen was almost 

 entirely excluded ; the limited gain in nitrogen amounting to about 2-3 mg. in 



