ASSIMILATION OF CARBON 201 



which we know to be rich in starch, and keep them for one 

 or two days in a moist, dark place, we shall find that the starch 

 will disappear, but an equivalent amount of sugar will appear. 

 The insoluble solid starch has changed into soluble sugar. 

 If we perform this experiment on a leaf still attached to the 

 plant, the starch will disappear, but an equivalent amount 

 of sugar will not be present in the darkened leaves. In this 

 case the starch has changed into sugar, but the sugar has 

 travelled away into the stem. In other words, carbohydrates 

 travel through the plant in the form of sugar. In the potato- 

 plant, when the sugar reaches the tubers it changes once more 

 into starch ; whereas, in the case of the beetroot, the sugar 

 is deposited as sugar in the root. 



Starch, Sugar, and Fats are food-substances. — We at once 

 ask of what use to a plant are starch, sugar, and fats which 

 have been manufactured by the leaves ? The question is easily 

 answered by reference to observations on seeds and plants 

 grown from them. We find that in a bean-embryo in the 

 seed there is much starch, and some proteid, as well as a little 

 cellulose. If we germinate this seed in darkness with the aid 

 of an inorganic culture - solution (page iqt), when the seed- 

 ling stops growing we shall find that it contains little or no 

 starch, but has relatively more cellulose, sugar, and proteids, 

 than was possessed by the embryo inside the seed. This 

 proves that the starch has been used up in the manufacture of 

 the cellulose framework and in building up new protoplasm. 

 In fact, the starch in the seed is changed to sugar and travels 

 to the growing parts of the shoot and root, and there acts as 

 food to the developing organs. Similar observations can be 

 made on potato-tubers which contain starch. Again, if the 

 seed contains much fat (or oil), as is the case with the castor- 

 oil seed, it is the oil which disappears and aids in the con- 

 struction of new plant-substance. All the chemical processes 

 involved in the building up of protoplasm from simpler sub- 

 stances are described under the name of assimilation. In the 

 process of assimilation of carbon, the earliest easily detectable 

 products in green parts are sugar, starch, or oil. These 

 substances may be consumed at once in the manufacture of 

 new protoplasm and new plant-substance ; or they may be 

 transported to some distant part of the plant and deposited 

 there- for future consumption. In the latter case they are 



