DEVELOPMENT OF PLANTS 



199 



parasitic algae closely related to these alga-like fungi. Their 

 parasitic habit and partial loss of chlorophyll clearly indicate that 

 they are in a transition state from the algae to the fungi. It is 

 therefore safe to state that in this group we have evidence of 

 the derivation of the fungi from the algae. 



79. Order b. Peronosporales. Downy Mildews and White 

 Rusts. — These form are parasites upon the higher plants espe- 

 cially and include two of the most destructive fungi, the potato and 

 grape vine blight. Phytophthora infcstans causes the potato rot. 

 This disease was widespread in the eastern United States in 1901, 

 causing the entire loss of the crop in some sections. It would 

 be difficult to imagine a pest more perfectly adapted to a destruc- 

 tive career. The tubular filaments of the fungus readily spread 

 through the tissues of the leaf, sending into the cells short lateral 

 branches called haustoria (Fig. 131), which absorb the cell con- 



Fig. 131. Hyphae of Pcronospora extending through the tissues of a plant 

 and absorbing food from the cells by means of haustoria, h. 



tents and thus supply the fungus with food. In severe cases this 

 produces a withering and decay of the leaf. The hyphae con- 

 tinue their growth into the stems and all parts of the plant, thus 

 causing the black discoloration of the potato and its early decay 

 in bad cases of infection. When 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 spor- 

 angia chance to fall upon leaves that are wet by dew or rain, the 



