406 Transactions of the Boyal Society of South Africa. 
certain amount of waste. Plants have marked powers of reconstruction 
in the cell, and therefore their metabolism is not connected with any such 
excretion of waste (chiefly urea) as is found in higher animals. The chief 
waste materials are Garbon and Hydrogen, which are oxidised by the 
atmospheric oxygen, under the influence of the protoplasm, into Carbon- 
dioxide and water. These waste products are removed from the vicinity 
of the cell concerned. There is thus instituted in each living cell an inter- 
change of gases, i.e., oxygen is introduced into the cell, and waste products, 
usually in the form of gases, are removed from it. This process, which 
includes both these, is known as respiration. Without a sufficient supply 
of food in the cell the molecular activity of the protoplasm would result in 
the diminution of the protoplasm, and eventually death. Food material is 
rapidly used up, and as waste goes on conjointly, the result is^ of course, a 
loss of weight unless the food supply is replenished. That there is such a 
loss in weight can be demonstrated in the following way. If some seeds 
are germinated for a few days in darkness it will be found that the total 
dry weight of the young plants will be less than the dry weight of the 
seeds (or control seeds) from which they grew. In fact the greater part 
of the food supplied to living cells is, as it were, wasted. Thus it has been 
calculated that the dry weight of a mould fungus is not greater than one- 
third to one-tenth of the organic food consumed. In mature animals the 
amount of food consumed does not increase the body weight, as the weight 
remains practically constant. A plant, however, is continually increasing 
in weight throughout its whole life. 
The nutritive substances required by the living cells of plants are some 
carbohydrate (usually sugar) and water wuth mineral substances in solution. 
Proteid material may also be a nutritive substance, but it is doubtful if 
proteids are supplied to growing cells. Proteids are probably manu- 
factured in all living cells from the carbohydrate and aqueous solutions 
supplied to them. The carbohydrate itself is produced on the plant in the 
green tissues as an additional product in the metabolism of the cell, in 
which the protoplasm uses different compounds from those used in colour- 
less cells. This additional metabolism, to which the name photo-synthesis 
has been given, involves the combined action of the living protoplasm, 
light and chlorophyll, by which carbon- dioxide from the air and water are 
probably first decomposed and then reconstructed into the carbohydrate, 
the waste products of the whole metabolism being oxygen, and presumably, 
carbon-dioxide. During the light the carbon-dioxide given off, as well as 
that of the air, can be absorbed by the cell with oxygen and carbon- 
dioxide as the end products, but as light fails the carbon-dioxide is not 
absorbed but passes off, whilst the evolution of oxygen ceases. With the 
first rays of light some carbon-dioxide is absorbed and the different 
metabolism commences. In the absence of Hght or chlorophyll, or both. 
