DEVELOPMENT OF PLANTS 223 



death of the infected parts. This pest caused the abandonment 

 of entire vineyards before the method of kiUing 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 ends 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 is usually formed singly, by means of a tube 

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

 spore, as in many of the algae, tides the plant over the winter, and 

 being set free by the decay of the surrounding tissues it germi- 

 nates 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 shall 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 motil- 

 ity in the male gametes and the formation of a tube to conduct 

 them to the female. For the same reason, the zoospores are re- 

 duced to light motionless spores that are developed upon elon- 

 gated hyphae that expose them to the air currents for distribution. 



