DEVELOPMENT OF PLANTS 209 



a hypha which enters the leaf and quickly spreads through its 

 tissues, sending into the cells short lateral branches called haus- 

 toria (Fig. 131), which absorb the cell contents and thus supply 

 the fungus with food. In severe cases this produces a withering 

 and decay of the leaf. The hyphae continue their growth into 

 the stems and all parts of the plant, thus causing the black dis- 

 coloration of the potato and its early decay in bad cases of in- 

 fection: This habit of many parasitic fungi of establishing them- 

 selves in those organs of the plant which live on from year to 

 year, is one of the most serious difificulties in combating these 

 pests. For with the renewal of growth of these organs the hyphae 

 spread and reestablish the disease. Thus the planting of infected 

 potatoes is sure to result in the appearance of the potato blight. 

 As soon as the mycelium has become well established in the leaf, 

 numerous branching hyphae extend out through the stomata and 

 form at their tips little sacs or sporangia (Fig. 132, A). The 

 formation of the sporangia is effected in a few hours, when they 

 drop off and are carried by the wind to other plants, where they 

 germinate at once, forming a tube that penetrates the leaf and 

 rapidly spreads the disease. If the sporangia chance to fall upon 

 leaves that are wet by dew or rain, the contents breaks up into 

 several zoospores (Fig. 132, B—E) which finally come to rest and 

 develop the characteristic tubular hyphae of the fungi This 

 behavior of the sporangium is doubtless a survival of the zoospore 

 stage seen in the algae. It is equally suggestive that definite 

 changes in the environment, as the dry air, causes the sporangium 

 to germinate as a non-motile spore, whereas the presence of 

 water causes the sporangium to produce several zoospores. The 

 potato blight is largely confined in our country to the north- 

 eastern states and usually it does not appear until the latter 

 part of July. It only becomes of serious importance in sultry 

 weather, when a short period of rain will result in the formation 

 of the sporangia and whole fields will be devastated in a few days. 

 Very similar in character is the grape blight, Plasmopara viti- 

 cola. This disease causes a browning of the leaves and the stunt- 

 ing of the fruit, which become brown or gray, and, finally, the 

 death of the infected parts. This pest caused the abandonment 

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