266 BOTANY PART i 



beet, may be connected with inulin. It is converted by the widely- 

 spread enzyme " invertin " into equal parts of dextrose and levulose. 



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 

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

 plant is able to form fats (glycerine esters of various fatty acids ; cf. 

 p. 30) from carbohydrates. Fats are always present in living proto- 

 plasm ; the general distribution of lecithin which is derived from fats 

 has already been mentioned. Fats occur in relatively large amounts 

 as reserve materials, but not in the assimilating foliage leaves. They 

 occur 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 carbo- 

 hydrate. 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. 



3. Albuminous Substances 



Albumen occurs in the storage places for reserve materials partly 

 in a crystalline and partly in an amorphous form. The crystals occur 

 free in the cytoplasm, nucleus, or in the chromatophores ; in seeds 

 they are" found especially in the aleurone grains, where they are 

 associated with globoids. The latter then contain Ca, Mg, and 

 phosphoric acid in an organic compound (cf. p. 31). 



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. When seeds rich in proteid 

 such as Eicinus, Pinus, 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} T 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 ammonia, 

 also products of decomposition containing sulphur and phosphorus, 

 and generally carbohydrates also. 



