SYNTHESIS, STORAGE AND BREAKDOWN 



125 



A somewhat different substance known as Reserve Cellulose occurs in certain 

 seeds (Lupin, Date) : and the cell-walls are in consequence much thickened 

 (Fig. 83). As its name implies, this substance is of nutritional rather than 

 structural significance. During germination it is converted into sugars under 

 the influence of an enzyme Cytase. 



Fats, although not carbohydrates, may be considered here. Fatty sub- 

 stances may figure in the make-up of protoplasm, as already mentioned, 

 and so be present in any living cell. It is in seeds and fruits that fats are best 



Fig. 82. 



Starch grains from the Potato. A. simple ; B, half 

 compound ; C, D, compound grains. c, organic 

 centre, or nucleus of formation. ( x 540.) (After 

 Strasburger.) 



Fig. 83. 



Cell-wall of a single cell of the endo- 

 sperm of Lodoicea, consisting of reserve 

 cellulose which forms the thickened 

 regions of the wall. ( x 400.) (After 

 Gardiner.) 



known and most abundant, forming in many cases the chief non-nitrogenous 

 reserve substance. This for example is the case in the seeds of the Castor-oil 

 plant {Ricinus) and in various nuts, — Brazil, Walnut and Hazel (see Appendix 

 B). The fats exist in the form of globules in the cytoplasm : they contain 

 the elements carbon, hydrogen and oxygen, and are organic salts produced 

 by reaction between fatty acids and glycerine, a basic substance. There is 

 no doubt that fats arise from sugars within the plant cell, though the steps 

 in the transformation are obscure. It is believed that fatty acids and glycerine 

 are separately formed from sugars, and the two then react together. During 

 the germination of fatty seeds a reverse change occurs, the fats being re- 

 converted into sugars. The well-known enzyme Lipase effects the formation 

 of fats from fattv acids and glycerine, and under other conditions it promotes 

 the reverse reaction. 



