DEVELOPMENT OF. PLANTS 201 



Very similar in character is the grape blight, P las-mo par a 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 

 of entire vineyards before the method of killing the fungus by 

 spraying the plants with copper salts was discovered. In both 

 of these pests and in allied genera the sporangia-bearing hyphae 

 are produced in such numbers as to cause a downy, mould-like 

 appearance on the leaves, thus accounting for their popular name, 

 Downy Mildews. 



In a related genus, Albugo, the sporangia are formed by the 

 repeated cutting off of the tips of the hyphae, as shown in Fig. 

 133, A. In this way, not one, but a chain of sporangia, are 

 formed from the end of the hyphae which do not project from 

 the leaf, but grow up in dense masses just under the epidermis, 

 producing glistening white blotches or blisters on the leaves. 

 This growth finally ruptures the epidermis when the spores are 

 scattered by the wind and germinate as in the preceding cases. 

 This fungus, known as white rust, is very common on mustards, 

 pigweed and other plants. 



The sexual reproduction of the Peronosporales is suggestive of 

 Vaucheria. Gametangia are cut off from the ends of the hyphae, 

 as shown in Fig. 133, B. The male gamete gains access to the 

 female gamete, which are usually formed singly, by means of a 

 tube as in Saprolegnia (Fig. 133, B, an). The thick-walled 

 gametospore, as in many of the algae, tides the plant over the 

 winter, and being set free by the decay of the surrounding tissues 

 it germinates in the spring, starting anew the life of the pest. 

 It may germinate directly (see Vaucheria) into the fungus or 

 zoospores are first produced, as in Oedogonium. 



These two orders are more suggestive of the algae than any 

 others that we will study and it is well to note the modifications 

 that have been induced in these plants as a result of their change 

 from aquatic to terrestrial conditions. Removed from the water, 

 special root-like organs and haustoria are evolved for the absorp- 

 tion of foods. The absence of water brings about a lack of 

 motility in the male gametes and the formation of a tube to con- 



