224 FORMATION OF PROTEINS 



from the leaves where 'carbon-fixation' is going on. Thus, the 

 starch in the cereal grains, in the tubers of potatoes, and in the 

 medullary rays and cortex of trees in winter, is formed from 

 sugars primarily manufactured in the leaves. 



Starch-grains formed by the leucoplasts are usually much 

 larger than those temporarily formed and stored in the allied 

 chloroplasts of the leaves. 



In certain seeds some of the carbohydrate reserve is stored in 

 the form of thickened cell- walls consisting of hemicellulose. 



The fats and fixed oils occurring in the seeds of flax, cotton, 

 and rape are non-nitrogenous reserve-materials, which are first 

 visible in the form of minute drops in the protoplasm ; the 

 small drops run together ultimately and form larger drops. In 

 some cases the fats and oils appear to be manufactured from 

 dextrose and other sugars, while in others they arise by the 

 conversion of starch. 



Asparagine, leucine, glutamine, and other amido-compounds 

 frequently form the chief store of nitrogenous materials present 

 in the cell-sap of tubers, roots and rhizomes of plants. With 

 increasing maturity of the root or tuber some of these 

 compounds are converted into proteids. In most ripe seeds 

 the nitrogenous reserve-material consists almost entirely of 

 proteins stored in the form of solid aleuron-grains and other 

 more or less amorphous masses : only a small proportion of 

 amido-compounds are present. 



It will be observed that the substances actually stored are 

 usually different in chemical constitution and solubility from the 

 organic materials transported into the cells where the storage is 

 proceeding. One form of sugar is changed into another after 

 entering into the cell or is utilised by the leucoplasts for the 

 formation of starch-grains ; ihe cell-sap, therefore, becomes less 

 concentrated in the particular sugar entering it, and a further 

 osmotic diffusion into the cell takes place. 



By these changes a continuous accumulation of reserve-materials 



