30 GENERAL BOTANY 



plant which has been out in the sun. Dip it first 

 into boiling water to kill it, then leave for a few 

 hours in alcohol, which will dissolve out the green 

 colour. When nearly white put it into a strong 

 solution of iodine, made by adding iodine to a solu- 

 tion of potassium-iodide in water. The leaf will 

 turn blue or black, because starch forms with iodine 

 .a blue compound. Drop a little of the iodine solution 

 on new white cotton cloth or paper (which usually 

 contains starch) and note the dark blue or black colour. 



Some plants, however, will not give this reaction 

 with iodine, because starch is not formed in their 

 leaves, the product of assimilation being sugar. But 

 most ordinary quickly grown plants, which have been 

 in the sunlight for a few hours, will give it. 



If you try this experiment with leaves from the 

 same plant which have been plucked in the early 

 morning, you will find that they do not turn such 

 a dark colour, showing that assimilation does not 

 go on during the night, and that the starch formed 

 during the day is passed down into the branches. 

 Being insoluble it has to be first converted into a 

 sugar and is passed down in that form to all parts 

 of the shoot and to the roots. 



It is only a certain part of the first formed pro- 

 duct of assimilation which is converted into starch 

 ..and sugar, and passed on as such to the rest of the 

 plant. The rest is made to combine with the mineral 

 substances taken up by the root to form with them 

 peculiar chemical compounds known as proteids. 



Proteids are, as we know, absolutely necessary to 

 animals as food, and they are just as necessary to 



