82 BOTANY 



up ; the carbon dioxide and some of the water disappear 

 and are replaced by a simple organic substance known 

 as formaldehyde, while a quantity of oxygen, equal in 

 volume to the carbon -dioxide, is set free. The oxygen 

 finds its way from the cells into the intercellular spaces 

 and passes out of the plant by way of the stomata. 

 Formaldehyde is thus the first organic product which is 

 formed; it is a gaseous body and probably is never 

 present in any but very small quantities, for it is almost 

 immediately transformed into a kind of sugar. The 

 manufacture of sugar is thus the first stage in the 

 preparation of the food of the plant. 



This construction of sugar cannot be carried out 

 without the application of energy. We are familiar, 

 from our ordinary experience of things, with the fact 

 that a machine cannot be made to do work without a 

 supply of energy. A steam engine cannot work without 

 the expenditure of a certain amount of fuel. Whence 

 then does the chloroplast obtain the energy which it 

 applies to sugar-making? The answer to this question 

 explains the necessity for the proper illumination which 

 we have spoken of as a condition of its activity. The 

 rays from the sun, which we speak of as the rays of 

 light, are absorbed by the green colouring matter of the 

 chloroplast. We can prove this by the use of an instru- 

 ment known as the spectroscope, which is an arrange- 

 ment of glass prisms. If we let a beam of white light 

 from the sun fall upon such a prism the rays of which it 

 is composed are bent or deflected unequally on entering 

 and leaving the glass, so that if they are allowed on 

 emerging to fall upon a plane surface they appear as a 

 broad band of light showing a series of colours ranging 

 in order from red to orange, yellow, green, blue, and 

 violet. If now we place a thin film of a solution of 

 chlorophyll between the source of light and the prism 

 we find all the rays do not reach the glass, so that the 



