THE FOOD OF PLANTS. 45 



which come within the region of errors of observation, are of 

 course to be disregarded. In assimilation, therefore, a quantity 

 of Oxygen is produced which is exactly equal in volume to the 

 quantity of Carbon dioxide decomposed. 



1 See Pfeffer, Arbeiten des bot. Institute in Wurzburg, Bd. 1, H. 1. 



2 See Holle, Flora, 1877. 



8 See Bunsen, Gasometrische Methoden, 1857, and Hempel, Gasanalytische 

 Methoden, 1890. 



14. Macroscopical and Microscopical Detection of Starch in the 

 Organs of Assimilation. 



In very many green plant structures amylum is produced as 

 the first easily visible product of assimilation. Ifc is therefore one 

 of the commonest tasks in plant physiology to detect this starch 

 in the organs of assimilation, and this can be done either macro- 

 chemically or microchernically. We will first consider the former 

 method. 



The simplest method of investigation, first used by Sachs x in 

 prolonged experiments, is as follows. We place the objects to be 

 tested for starch (leaves of Tropasolum, Helianthus, Solanum, or 

 Phaseolus are very suitable) for a few minutes in boiling water, 

 and then transfer them to very strong alcohol at a temperature of 

 60 C. If we use a fairly large quantity of the alcohol, the chloro- 

 phyll of the leaves is generally very quickly and completely taken 

 up, and in a few minutes the leaves become colourless. They are 

 now placed in a solution of Iodine. This is prepared by dissolving 

 a fairly large quantity of Iodine in strong alcohol, and then pour- 

 ing the solution into distilled water till the fluid is about the 

 colour of dark beer. We may also satisfactorily employ a solution 

 of Iodine in Potissium iodide solution. The leaves are left in the 

 Iodine solution for half an hour, an hour, or, if necessary, for 

 several hours, till they undergo no further change of colour. We 

 then remove them from the solution with the forceps, and lay 

 them in a porcelain dish containing water. If starch is completely 

 absent, the Iodine-saturated leaves are light yellow or buff- 

 coloured. Small quantites of starch are indicated by darkish 

 coloration of the leaves ; large quantities by deep black color- 

 ation. If Iodine-saturated leaves, rich in starch, are left for 

 several hours on a plate containing water, they often assume a 



