NUTRITION OF PLANTS 19 



the endosperm, and not the cotyledons, provides the requisite 

 nutrition for the young plant. 



NUTRITION OF PLANTS 



In the youngest stages of its life, as has been shown, the plant 

 is supplied with nourishment from the food stored up in the seed 

 for that purpose. By the time the roots and leaves have been 

 formed, this supply becomes exhausted, and the plant must now 

 obtain its food from the soil and the air. Through their delicate 

 root-hairs the roots absorb from the soil various chemical salts in 

 solution, which are transported upwards through the stem and 

 distributed to the leaves. The acid sap of the root, together with the 

 CO 2 (carbon dioxide) gas, probably assists the moisture in dissolving 

 the mineral matter in the soil. The leaves imbibe from the atmos- 

 phere, through their stomata, fcarbon dioxide, the carbon of which 

 enters into combination with the substances absorbed by the roots 

 to form sugar and then starch, and also albumen matter. The 

 process of forming starch by the agency of chlorophyll (the green 

 colouring matter in the leaves and green parts of plants) is called 

 assimilation or, to use the more modern term, photosynthesis. This 

 can only take place under the influences of light and air, the latter 

 supplying the CO 2 and the former the energy by which the 

 carbon is split from it. During sunlight the stomata of the leaves 

 are continually giving off oxygen which has been separated from 

 the CO' taken in. the carbon itself being retained by the leaf. 

 In darkness no fixation of carbon can take place, as light is 

 necessary for the splitting of the CO 2 , so no starch is formed 

 and at the same time no oxygen is evolved. Consequently the 

 process of respiration, which is always going on, becomes evident 

 through the evolution of CO 2 In sunlight this is masked as the 

 CO' 2 formed in the leaf by respiration is immediately concerned 

 in assimilation. 



As the saline matter is carried up from the soil through the 

 plant in extreme dilution, the excess of water thus absorbed has to 

 be got rid of in the leaf. This passes out through the stomata in 

 the form of water vapour and the process is known as transpiration. 

 It is important therefore to distinguish clearly between the three 

 processes of assimilation, respiration, and transpiration. 



Food of plants. The main elements necessary in the food 

 of plants may be said to be hydrogen, oxygen, carbon, nitrogen, 

 sulphur, phosphorus, potassium, magnesium, calcium and iron. 



