436 



A TEXTBOOK OF BOTANY 



[Ch. X 



thick walls and become resting spores which later produce new 

 mycelia. They spread by their zoospores, or are carried by 

 water currents and movements of their hosts, without any 

 special adaptations of their own. Some 55 species are known. 



Order 2. Peronosporales : the Blights, or Downy 

 Mildews. These are parasites which live in the tissues 

 of the higher plants, and there produce some of the most 

 serious plant diseases. About 100 species are known. 



An important and typical form is the Blister Blight or 

 White Rust (Fig. 303), which attacks members of the 



Fig. 303. — Albugo Candida. 



Left, cross-section of a leaf, showing mycelium, with conidia forming 

 in the blister ; x 60. Next, swarm spores ; X 400. Right, oogonium con- 

 taining one egg cell in process of fertilization through a tube from an antherid- 

 ium ; X 400. (After Bergen and Davis, Principles of Botany, and other data.) 



Cruciferce or Mustard family. Its mycelium runs every- 

 where through the intercellular spaces of the host, and sends 

 short haustoria into the living cells themselves, in which 

 are produced those typical toxic effects which cause damage 

 and death (page 85). Asexual reproduction occurs through 

 formation of many conidia (spores) on the ends of hyphal 

 branches collected just under the epidermis. Thus is 

 produced a white blister, from which, when it bursts, the 

 spores are scattered by wind. These germinate in wet 

 weather on the surface of leaves of suitable kind to which 



