344 THE LIFE OF THE PLANT 



water or some kind of extract is allowed to stand in the light, 

 a green deposit soon appears on the walls of the vessel. Now- 

 adays we know that this deposit is composed of microscopic 

 plants, of algae ; but in Priestley's time this fact was not known, 

 and the deposit was even known as ' Priestley's matter.' 

 Priestley was able to prove that this matter gave off oxygen. 

 This experiment already showed that the green substance 

 decomposed carbonic acid, even outside the leaf or stem, and 

 that it was precisely to this green substance that these functions 

 were due. Other doubts, however, rose later on. There are 

 plants which are not green in Nature, and yet they also decompose 

 carbonic acid. Such are the numerous plants with red, black, and 

 other coloured leaves, which are more and more gaining a footing 

 in our gardens and greenhouses ; such are also the brown and 

 red weeds growing at the bottom of the sea. In the former the 

 matter has been easily explained. The variegated colour in such 

 plants depends on bright solutions which exist in their cell-sap, 

 and which conceal the green chloroplasts. These are easily seen 

 under a microscope, and can be also revealed in the following way. 

 We have only to dip a red or almost black leaf of Coleus, or some 

 other plant with similarly coloured leaves, into dilute sulphurous 

 acid, and it immediately turns green. This depends on the fact 

 that in decolourising the red pigment sulphurous acid does not 

 affect the chlorophyll. It was rather more difficult to prove the 

 presence of chlorophyll in seaweeds. It was impossible to find 

 the green grains in them even under a microscope ; they were 

 all brown or red. Chemistry, however, showed that the green 

 chlorophyll is concealed behind another substance. It is easy 

 enough to observe this fact simply by walking by the seaside. 

 Weeds cast on the shore very often manifest as they decompose 

 all shades of colour from their natural colour to green. This 

 is because in dead plants the more brightly coloured substances 

 are washed away by water, while chlorophyll remains insoluble. 

 Thus, even here the decomposition of carbonic acid takes place 

 only in parts which contain chlorophyll granules. This rule 

 has no exception. As has been already said, we must see in a 

 chloroplast the apparatus, the mechanism to which the energy 

 of the sun is applied. It was very interesting to test by means 

 of an experiment the truth of this hypothesis, and to see whether 

 the decomposition of carbonic acid actually did take place at 



