DEVELOPMENT OF PLANTS 221 



daily and include two of llic most destructive fun^q, the ix>tato 

 and the grape vine bliglit. PJiytophthora infestans causes the 

 potato rot. This disease was widespread in the eastern Lnited 

 States in 1901, causing the entire loss of the crop in some sections. 

 It would be difficult to imagine a pest more perfectly adai)ted to a 

 destructive career. Let us begin the life history with the j^'crmi- 

 nation of a spore which has fallen upon a leaf. This spore ff)rms 

 a hypha which enters the leaf and cjuickh- 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 >ear to 

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

 pests. For with the renewal of growth of these organs the h>phae 

 spread and reestablish the disease. Thus the planting of infected 

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

 conditions are favorable. 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 b>- the 

 wind to other plants, where they germinate at once, forming a 

 tube that penetrates the leaf and rapidly si)reads the disease. 

 If the sporangia chance to fall upon leaves that are wet b\' 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 tlie environment, 

 as the dry air, causes the sporangium to germinate as a non- 

 motile spore, whereas the presence of water causes the sp(»rangium 

 to produce several zoospores. The potato blight is largely 

 confined in our country to the northeastern states and usually it 



