220 



FARM FRIENDS AND FARM FOES 



through its lusty stalks to the little tubers at the ends of 

 the underground stems, there to be stored in cells as 

 starch. All over the surfaces of the leaves are the tiny 

 openings or breathing pores through which the air gets to 

 the leaf cells and through which the surplus water may es- 

 cape as vapor. The plant is 

 healthy and vigorous and ap- 

 parently enjoying its existence. 

 But one day a breeze springs 

 up that carries particles of dust 

 from field to field. In the dust 

 are many summer spores of the 

 mildew fungus, some of which 

 are caught on the surface of 

 the potato leaves. The breeze 

 is followed by a summer shower 

 that soon passes, leaving the 

 foliage wet with glistening 

 drops that reflect the hues of 

 the rainbow as the sun comes out again. 



In one of these drops a Downy Mildew spore is held. 

 It needed only the moisture to cause it to produce curious 

 swarm spores that swim about in the water and finally 

 germinate, each by sending out a slender tube. The tube 

 lengthens, the end creeping along till it reaches one of the 

 open breathing pores. It enters this and then begins to 

 grow rapidly, sending branching threads between the leaf 

 cells in all directions. These threads form the mycelium 

 of the fungus. They send into the cells short suckerlike 

 branches through which the contents of the cells are 

 absorbed. Thus the threads of the fungus penetrate 

 through all the tissues of the leaf. Finally they run 

 through the petiole to the main stalk and perhaps down 



Potato Leaf affected with 

 Late Blight 



