i66 



Chapters in Modern Botany 



of water, those which were rich in starch assume the 

 characteristic blue colour. Of this test for starch we can 

 make many other uses ; by growing plants in darkness, or 

 in an atmosphere without any carbonic acid gas, we can 

 show that starch is not formed in the leaves unless they are 

 green, unless they are illumined, unless they are growing 

 in a medium where carbonic acid gas is available. 



The leaves are able to manufacture many other organic 

 substances besides starch, and their presence is also of 

 course demonstrable by experiment, but starch is com- 



FiG. 8. — Apparatus of Bonnier and Mangin, for analysis of gases given off by 

 plants, c, Reservoir of Mercury ; in ;//, graduated tube ; v, screw of 

 piston, which enables the gas to be drawn into the graduated tube and 

 measured ; and after it the potash and pyrogallic solutions successively each 

 being thoroughly brought into contact with the gas in the dilatations a. 



monest and simplest, and may well serve as a type of the 

 products built up by the life of the leaf. 



It is also very important to show by experiment, by 

 the analysis of plant ashes, and by growing seeds in 

 artificially-prepared solutions, that certain salts, especially 

 nitrates and sulphates, are essential to the health of the 

 plant and to the leafs manufacture of complex organic 

 substances. The successive omission of each particular 

 ingredient gives rise to some recognisable defect of growth, 

 e.g. when iron is absent we have a blanched plant, while 



