200 NUTRITION 



Green parts wMch do not produce starch.— Some plants 

 produce little or ho starch in their green leaves. The onion, 

 for instance, produces sugar. Some other plants produce oils 

 at the expense of the carbonic acid. 



Why is Light essential to the production of organic matter 

 at the expense of Carbonic ASid? — If we apply a light to 

 organic material (say wood) we''li;now that it will burn. We 

 make the burning wood (or leaves or coal) do work by setting 

 it to drive an engine. The work'is done because heat is given 

 out by the combination of oxygen with the organic material. 

 The oxygen and organic material combine and produce 

 compounds including water, carbonic acid, and ammonia. 

 Now, if we want to make these latter substances once more 

 form organic substance, we must remove the oxygen with which 

 their carbon is combined. In order to remove the oxygen it 

 is necessary to restore the heat (or its equivalent) which was 

 given away when the organic matter combined with the 

 oxygen. In other words, to tear the oxygen of carbonic acid 

 apart from the carbon, force must be applied ; and this force 

 (or, more properly, energy) is supplied to the leaves in the 

 form of sunlight. The green part of a plant is a machine for 

 , collecting and holding sunlight, by the aid of which to force apart 

 the oxygen and carbon of carbonic acid, with the object of build- 

 ing up organic matter (starch, sugar, fats, etc.). Put into 

 rough words, a leaf is a "trap to catch a sunbeam." 



The transport of Carbohydrates in a Plant. — Starch and 

 sugar are not only found in the green parts of plants, but they 

 also occur in parts which have not been exposed to light and 

 which do not contain chlorophyll. For instance, the subter- 

 ranean tubers of the Pptato contain much starch ; the fleshy 

 roots of the Beet and Dahlia are rich in sugars. Starch and 

 sugar can appear in these underground parts even when the 

 plant has obtained its carbon solely from carbonic acid. Yet 

 we have learned that if plants be compelled to obtain their 

 carbon from carbonic acid, starch and sugar are formed only 

 in the green parts exposed to light. This proves to us that 

 the starch or sugar in the roots or tubers .must have been 

 derived from the organic material manufactured in the green 

 parts ; it is therefore evident that organic material can travel 

 inside the plant. This is easily shown by simple experiments 

 on leaves which manufacture starch. If we pluck some leaves 



