340 THE LIFE OF THE PLANT 



energy incessantly accumulated. These organisms are plants. 

 The vegetable world is a kind of storehouse, where the sun's 

 rays become arrested and stored for further use. The physical 

 existence of mankind depends on this economical solicitude of 

 Nature, and a single glance at our luxuriant vegetation involun- 

 tarily provokes the sensation of prosperity.' 



Thus it is that in the dissociation of carbonic acid and the 

 formation of organic plant substance we have essential con- 

 ditions for any technical process. We possess in the sun- 

 light a motive power ; in the plant — a machine to which the 

 motive power is applied ; in the carbonic acid — a raw material ; 

 in the organic matter of the plant — the manufactured product. 



Let us investigate more closely the inner mechanism of this 

 process. 



Let us study first of all the source of energy, the sunbeam. 

 We know that the sunlight, like any other white light, is not 

 homogeneous ; we know that it consists of many heterogeneous 

 rays, differing, among other things, in their colour. Rays of 

 seven different colours are generally discriminated ; they are 

 the colours of the rainbow — red, orange, yeUow, green, blue, 

 indigo, violet. This decomposing of a colourless ray into its 

 seven component colours is best performed by means of a glass 

 prism. If a small aperture is made in a shutter facing the sun, 

 the sunlight in passing through this aperture will produce on 

 the floor an image of the sun in the form of a round patch. Now 

 if we place a prism with its edge downwards in front of the 

 aperture, the image wiU move on to the wall ; but instead of a 

 round patch we shall obtain a band showing the seven colours 

 of the rainbow just mentioned : the band will be red at one edge 

 and violet at the other. This rainbow band is called a spectrum. 

 Whenever a ray of white light falls upon the surface of a body 

 of any kind, it becomes partly or entirely absorbed. If all 

 the rays become absorbed by the body, its surface appears 

 black ; if all the rays are reflected in equal measure its surface 

 appears white. If some of the rays are absorbed and others 

 reflected, the body acquires the colour of those rays which are 

 reflected from the body and strike our eye. The same holds 

 true with regard to transparent bodies. If the body absorbs 

 all the rays it is not transparent, it is opaque ; if it lets all the 

 rays pass through it, it is entirely transparent, and as colour- 



