THE DOWXY MILDEW 



83 



are attacked their grovv^tli is cliecked^ and they gradually 

 turn grayish or brown. 



A good example of the parasitic fungi affecting 

 higher plants is furnished by this downy mildew of the 

 grape. It is a minute, parasitic plant that develops 

 at the expense of the tissues of the grape, thus causing- 

 blighting of the leaf and decay of the fruit. It attacks 

 all the green parts of the yine, including the young 

 shoots, as well as the leaves and berries, and, like other 

 fungi, reproduces by means of spores. 



When one of these spores falls upon a leaf where 

 there is sufficient condensed moisture, its contents divide 

 into a number of distinct 

 particles, which escape 

 through an opening in the 

 spore-wall. Each of these 

 particles moves about in 

 the drop of water on the 

 leaf for a few minutes, 

 then comes to a standstill fig. 40. section of leaf snowr^a 

 and germinates by sending mycelium of fungus, magnified. 

 out a little tube — somewhat as a kernel of corn in moist 

 soil sends out its germinating radicle — and this tube 

 penetrates the epidermis, or skin of the leaf. Once 

 inside, the tube continues to grow, pushing about be- 

 tween the cells of the leaf, and forming the mycelium, 

 or vegetative portion of the fungus, which may be 

 likened to the roots of the higher plants. As there is 

 little nourishment to be obtained between the cells, this 

 mycelium develops minute processes, which push through 

 the cell walls and absorb the cell contents. A small sec- 

 tion of an affected leaf, greatly magnified, is represented 

 in Fig. 40, the unshaded double-walled spaces represent- 

 ing the leaf cells, the shaded part between the walls the 

 mycelium of the fungus, and the projections marked 

 a, a, the processes, or suckers, that penetrate the cells^ 



