SYNTHESIS, STORAGE AND BREAKDOWN 131 



parts. In addition we have the upward movement of salts from the 

 roots to the aerial parts of the plant, to which the term translocation is 

 sometimes extended. This has already been discussed in Chapter YII. 



The leaf is the organ in which the synthesis of carbohydrates and 

 probably of proteins is chiefly located, and provision must be made 

 for the conveyance of these manufactured materials from the leaves 

 to the apices of root and shoot, or to regions where there is cambial 

 activity. Further, the rate of synthesis of materials in the leaf is 

 usually in excess of that demanded by current growth. The balance 

 of material is deposited in various parts of the plant in the form of 

 carbohydrate (starch, sugar, inulin, reserve cellulose), fat, or protein 

 and may serve for future use. Any parenchymatous tissue may serve 

 for storage. For instance, in trees which lose their leaves in autumn 

 and form a new suit of them in the spring, the material for these is 

 prepared in the previous season, and stored in the medullary rays, 

 wood parenchyma, and cortex of the trunk and branches. But in 

 many cases, especially in herbaceous plants, storage is effected in parts 

 which may become greatly distended, as in the turnip, carrot, and 

 potato (see also Chapter XL). Seeds habitually contain stored 

 material, the product of the activity of the parent plant. They meet 

 the needs of the seedling in the early stages of growth. Storage, in 

 one form or another, is then a common phenomenon in plants. It 

 involves the translocation of the materials from the point of produc- 

 tion, usually the leaves, to the point of storage. 



The translocation of manufactured materials out of the leaf to 

 regions of growth and storage proceeds constantly, but is most 

 easily detected when the leaf is in darkness. Synthetic processes 

 are then stopped or slowed down. Suitable tests show that both 

 the carbohydrates and protein of the leaf decrease in amount over- 

 night, chiefly as the result of translocation. Thus, application of 

 the iodine test will show that the starch content of the leaves of a 

 given plant is much less in the morning than in the previous evening, 

 while if the plant is kept in darkness for a further period all the 

 starch will disappear. 



At a later date the reserve substances stored in the various tissues 

 mentioned above will be required for growth and undergo trans- 

 location, as already noted. At all times materials to be translocated 

 must be brought into a soluble form, i.e. they must be mobilised. 

 Materials undergoing translocation are mostly crystalloidal in 

 nature, though there may also be some movement of colloidal 

 materials. There is evidence that sucrose is the usual form in which 



