DEVELOPMENT OF PLANTS 211 



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 ghstening 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 motility 

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

 to the female. For the same reason, the zoospores are reduced 

 to light motionless spores that are developed upon elongated 

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

 ability of these spores, or sporangia, to produce zoospores is 

 doubtless the survival of a trait inherited from their algal ances- 

 tors. 



80. Order c. Mucorales or Black Moulds. — These are among 

 the most common of the fungi and they are almost sure to appear 

 upon any cooked food or decaying matter that is exposed to the 

 air even for a very short time (Fig. 134). The mycelium has 

 practically the same structure as noted in the preceding groups, 

 but the black mounds have lost all motile reproductive bodies and 



