CHEMICAL AND PHYSIOLOGICAL STUDIES. 161 



actual combination of the water with protoplasm (comparable to 

 the formation of H^SO^ : xHfi when water is added to sulphuric 

 acid). This is an endothermic reaction, and is a source of energy 

 to the cell. In this case, also, the heat evolved during respiration 

 must be taken into account. On the other hand, the action of 

 enzymes mentioned above is in the main one of breaking down of 

 complex compounds into simpler bodies (hydrolysis), and, as such, 

 often requires heat from outside — viz., it is an exothermic re- 

 action, and involves a loss of energy to the cell. The cellulose and 

 woody framework of a plant represent a store of potential energy, 

 whilst the oxidative processes in the cell liberate an amount of 

 kinetic energy, which appears in the form of heat. 



B. Details of Vital Processes. 



Having now obtained an outline of the main vital processes to 

 be considered in the cell, it is necessary to examine in detail a 

 few of the more important of these, and, where possible, try 'to 

 elucidate some of the intermediate stages in the formation of the 

 essential food-substances elaborated by a cell from the raw- 

 material supplied. 



In this respect the following will be described : — 



i. Starch and starch-formation. 



ii. The relation existing between ehlorophyll, light, and the 

 assimilation of CO2 and H2O. 



iii. The formation of elaborated nitrogenous food. 



iv. The eell-sap and the meelianics of sap-eonduetion. 



V. The evolution of oxygen during assimilation, and of carbon 

 dioxide and water during respiration. 



vi. The assimilation of carbon dioxide and water from the 

 surrounding medium. 



vii. Variations of protoplasmic activity under different con- 

 ditions, especially those concerned with growth in light Of Varying 



refrangibility, and the effect of gravity and other physical 

 agencies upon growth. 



viii. The production of heat, light, and changes in electrical 

 potential in cells of plants; action of electric currents upon 

 cytoplasm. 



Each of these must be considered in detail; v. and vi. 

 include experiments which demonstrate the processes mentioned. 



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