ECT. II PHYSIOLOGY 237 



2. The Fats 



Though we are unable to manufacture the reserve carbohydrates 

 mentioned either from dextrose or levulose, we can understand that 

 it is as easy for the plant to build them up as to break them 

 doAvn. It is much more difficult to understand in what way the 

 plant is able to form fats (glycerine esters of various fatty acids) 

 from carbohydrates. Fats are, as a rule, absent from the assimilating 

 cells ; they occur on the other hand in large amount in many 

 ripe seeds, where they are formed at the expense of carbohydrates. 

 At germination they are decomposed by the enzyme lipase into 

 fatty acids and glycerine. The fatty acid is capable of passing 

 through the water-saturated cell wall more readily than the fat, but 

 does not usually travel as such for any considerable distance in the 

 plant ; it is usually quickly converted into a carbohydrate. A fatty 

 oil sometimes occurs in the succulent portions of fruits, e.g. in the oil- 

 palm and the olive, and then does not enter again into the metabolism 

 of the plant. Exceptionally it occurs in the wood and rind of some 

 of our trees in winter, disappearing again in the spring ; in this case 

 it apj)ears to be a means of protection against the cold. 



3. Albuminous Substances 



The products of the hydrolytic breaking down of albuminous 

 substances are mainly amino-acids, the wide distribution of which in 

 the plant has already been referred to. AVhen seeds rich in proteid 

 such as Eicimis, Pinvs, etc. are germinating, the abundant amino- 

 acids may be regarded as derived from the proteid. Amino-acids 

 occurring in other situations may have arisen in the synthesis of 

 proteids. The proteid-molecule does not produce at once or ex- 

 clusively amino-acids ; the breaking down of the ver}' large molecule 

 is a gradual one, in which the bodies which appear first have many 

 properties in common with proteids ; first comes albumose, then 

 peptone, and only then amino-acids. With the latter appear hexon- 

 bases and ammonia, also products of decomposition containing sulphur 

 and phosphorus, and generally carbohydrates also. 



The breaking down of proteids takes place under the influence of 

 enzymes which resemble the trypsin formed by the pancreas, and 

 are therefore termed tryptic enzymes. They act best in a weakly 

 acid solution. 



The decomposition products of albumen quickly undergo changes in tlie plant 

 (•'2), and therefore the mixture of nitrogenous organic compounds wliich one obtains 

 from a plant kept in the dark is not identical with the products of the hydrolysis 

 of albumen outside the plant. In the plant syntheses take jilace after the primary 

 decomposition, and these lead to he formation of such substances as amides, the 

 most widely spread of which is asparagin. These dominate in Gramineae and 



