TWO EPIDEMICS OF POTATO BLIGHT AND ROT. IO7 



the copper sulphate or blue vitriol used in its preparation. The 

 lime when added makes a very finely divided, but temporarily 

 insoluble compound suspended in water which we call bordeaux 

 mixture. The lime forms with the copper salt, compounds 

 which slowly become soluble and makes an adhesive mixture, 

 when dry, which slowly gives up the copper — fast enough to kill 

 the fungus spores but not fast enough to damage the potato 

 leaves. A very weak solution of copper sulphate would have 

 the same effect but it would have to be applied daily in rainy 

 weather. Spraying then serves a two-fold purpose, primarily 

 to cover the entire healthy leaf with a thin protective film of 

 bordeaux mixture which is constantly giving up minute quan- 

 tities of soluble copper salts which kill all blight spores which 

 find lodgment thereon before they can germinate and enter the 

 leaf, and secondly if the blight is already started the spray may 

 at the time of application kill many millions of spores which are 

 ripe and ready for distribution. 



The relation of the fungus to the disease is better understood 

 by reference to Fig. 15, page 169. At the left is seen a section 

 of an infected potato leaf, highly magnified. It will be seen that 

 the leaf is made up of many irregularly shaped units or cells. 

 Those above and below being flattened and forming a protec- 

 tive layer. In the lower layer certain lip-shaped openings are 

 seen. These are the breathing pores or stomata through which 

 the fungus most easily enters the leaf. It is possible, however, 

 for the germ tube of the fungus to penetrate directly through 

 the cell wall as shown in the illustration. Running between the 

 cells of the leaf may be seen the threads of the fungus. In 

 the drawing these fungus threads, though naturally nearly color- 

 less, have been colored to make them stand out more distinctly. 

 Projecting downward from the underside are two different 

 appearing bodies. The tapering, pointed ones are natural leaf- 

 hairs. The others, thread-like and branched and bearing the 

 knob-like bodies, are the fruiting organs of the fungus, each 

 little knob being a spore. The top row at the right shows the 

 stages in the development of a spore. After the spore is 

 formed it may divide up into from 6 to 16 little swarm spores, 

 each provided with two little hair like processes by which it is 

 enabled to swim around in drops or films of rain or dew. Later 

 these lose their swimming organs and begin to throw out a germ 



