380 THE FOOD OF PLANTS 



last century, Darwin was the first to study the special peculiarities of these 

 plants more closely, and the fact that we are simply dealing with a special mode of 

 nutrition was first clearly indicated by Pfeffer 1 , although the existence of car- 

 nivorous fungi should have sufficed to show that this mode of nutrition is by no 

 means a new development. 



SECTION 66. Nutritive Value of Different Carbon-compounds. 



That different carbon-compounds have not the same nutritive value 

 is well known, and this value moreover differs in different cases, so that 

 a substance which forms a good nutritive medium for one plant, may 

 be a poor or imperfect one for another. This is essentially a subject 

 for experimental investigation, but no satisfactory explanation can be 

 given even of the few facts already observed, owing to the incompleteness 

 of our knowledge concerning the inherent nature of the vital mechanism. 

 It is however certain that the various changes which the food-material 

 undergoes must not only yield a supply of energy but must also produce 

 all those substances which are employed in growth, for if a single part of 

 the intricate vital mechanism fails, or if a single essential metabolic product 

 cannot be formed, growth and development must always be partially 

 or entirely inhibited. Thus Penicillium glaucum and many other fungi 

 can grow when sugar is their only source of organic food, because they 

 are able to construct all the essential forms of proteid from sugar and 

 inorganic nitrogen compounds. When this power is absent development 

 is impossible unless a suitable organic nitrogen-compound is supplied. 

 Many fungi and bacteria require peptone and other proteids, while other 

 species can grow when supplied with asparagin (Sect. 64). 



Not only may the absence of a single essential compound finally 

 cause a stoppage of the general metabolism, but the mere disuse of par- 

 ticular functions or organs may induce various disturbances which operate 

 more or less inimically upon the plant. This is always apt to occur under 

 abnormal nutritive conditions, as for example when a chlorophyllous plant 

 is directly fed with sugar, or when an attempt is made to cultivate a 

 parasite in a nutrient solution. Similarly the physiological combustion of 

 particular carbon-compounds may be one of the conditions necessary 

 for the existence of certain organisms, and these may be unable to pro- 

 duce the substances in question even from otherwise suitable organic 

 food-material. 



Probably most of the lower plants require two organic food-substances 



1 Pfeffer, Landw. Jahrb., 1877, fid. VI, p. 969. 



