MISCELLANEOUS CROPS. 143 



(of infested plants), 'whicli become marked with elongated 

 white spots. The mycelium of C. candidus lives amongst 

 the intercellular spaces of the host-plant ; and, like that 

 of Peronospora parasitica, is provided with haustoria 

 (suckers). When a white spot or sorus of Cystopus is 

 examined under the microscope, chains of oval-sha,ped 

 conidia are seen rising from an embedded mycelium (I'ig- 

 47 A). The granular protoplasm of each conidium gives 

 rise, on a damp surface, to a number of secondary spores 

 (zoospores) provided with cilia (Fig. 47 B). After a time 

 the cilia disappear. The zoospores germinate and repro- 

 duce the mycelium and its accompanying conidia and 

 secondary spores. By the continual reproduction of the 

 mycelium and zoospores, the disease is propagated from 

 leaf to leaf and from plant to plant. The mycelium of 

 Cystopus also gives rise to antheridia and oogonia (Mg. 

 47 C), or male and female organs respectively. After 

 fertilization, each oogonium produces oospores, i.e., rest- 

 ing-spores (Kg. 47 D). Before the oospores are ripe 

 several months must elapse, and the ripening process goes 

 on within the tissues of the host-plant. In fact, the fer- 

 tile oospores hibernate until the following spring, when 

 they " germinate on the ground during wet weather." 

 The protoplasm of the oospores gives rise to zoospores 

 (Fig. 47 E and F), which ultimately reproduce the " white 

 rust " mycelium, and its accompanying conidia on the 

 young leaves, etc., of cabbages and allied plants. " No 

 doubt the little motile zoospores are carried through 

 moist air by currents of wind, and distributed in every 

 direction throughout the country." 



Prevention.— {I) Destroy, as far as possible, cruciferous 

 weeds, especially shepherd's purse. (2) Destroy all in- 

 fested stumps, etc. (3) Rotation of crops tends to lessen 



