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Transactions of the Boyal Society of South Africa. 
has wonderful regulative power. Two plants grown under the same 
conditions of light, chlorophyll, &c., may produce quite different carbo- 
hydrates, whilst the number of proteid substances already known is very 
great. 
This metabolism, of course, requires a large amount of energy. It is 
usually stated that all the energy required by living cells has been 
obtained primarily from the sun's rays, i.e., Pfeffer's "Physiology of 
Plants," vol. i., p. 297: "Both animals and plants derive their energy 
either directly or indirectly from the sun's rays which are converted into 
chemical energy by the agency of the chlorophyll apparatus. When a 
piece of coal is burnt a portion of the sun's radiant energy is liberated 
which was stored up in the form of potential energy many years ago, so 
that every steam engine is directly or indirectly driven by the sun." The 
whole of this statement I hope to show to be altogether erroneous. 
Energy may be defined as the amount of heat given out by a substance on 
being burnt or oxidised ; that is, in all combinations of oxygen with other 
substances heat is usually liberated, especially in any combinations with 
carbon compounds. Heat or energy is measured in units called calories. 
A calorie is the amount of heat required to raise one gram of water 
through one degree Centigrade. In seeking for the energy available for 
the metabolism of the protoplasm two kinds of cells have to be considered 
— green cells and colourless cells. In green cells carbon-dioxide, water, 
and the protoplasm itself are being continually decomposed, with re- 
construction chiefly of some carbohydrate. Now in order to break down 
a substance such as carbon-dioxide energy is required. The protoplasm, 
by the agency of the chlorophyll, undoubtedly makes use of the sun's 
rays to effect this. The energy required to break down a molecule of 
carbon-dioxide is the same as that given out when a carbon atom is burnt. 
This energy has been calculated in calories and may be represented in the 
chemical equation thus : — 
0 4-03 = 00,4-97,000 cal. 
This is a gram-molecule equation and means that when 12 grams of carbon 
unite with 32 grams of oxygen, 44 grams of carbon- dioxide are produced 
w^ith the liberation of 97,000 units of heat. Or again, if about half a 
milligram of carbon is burnt, one cubic centimetre of carbon-dioxide is 
formed with the liberation of about 4 units of heat. A similar equation 
for the formation of water is : — 
H, + 0 = H30 + 68,400 cal. 
The reverse of such an equation is true ; that is, 68,400 calories are 
required to decompose a gram-molecule of water. It is usually stated 
that plants are enabled by the processes going on in their leaves to store 
