PERONOSPORALES 187 



obligate. The mycelium is strictly intercellular except for the 

 production of small knob-like haustoria which lie just within 

 the host cells. The sporangiophores are short, broadly clavate, 

 and usually unbranched (Fig. 69). They are developed in a 

 compact group just beneath the epidermis, and form there a 

 definite palisade layer. At the tip of each sporangiophore 

 sporangia are abstricted in basipetal succession, and remain 

 attached forming a chain. The outward growth of these chains 

 exerts sufficient pressure to rupture the epidermis, and an open 

 sorus resembling that of the caeoma stage of certain rusts, except 

 in color which is here white, results. Early investigators called 

 the members of the family "white rusts." A superficial resem- 

 blance to the rusts is noted not only in the aspect of the sori, but 

 also in the striking similarity of the chains of sporangia to chains 

 of aecidiospores. Even an intercalary cell seems to be present 

 between each pair of sporangia in the chain. This appearance 

 is, however, an illusion resulting from the gelatinization of the 

 outer layer of the walls of the sporangia at their point of contact, 

 with the consequent formation of a small gelatinous disc between 

 them. Complete gelatinization of the disc results in the falling 

 apart of the sporangia. True intercalary cells such as exist in 

 the rusts are absent. Though the sporangia tend to hang 

 together for a time in rather long chains, they finally separate 

 and form a dry white powder easily disseminated by the wind. 

 The sporangium germinates by means of swarmspores. 



The sexual organs are formed in the intercellular spaces of 

 the host. The oogonium is globose and develops from a terminal 

 or intercalary swelling of the mycelium. Its contents are clearly 

 differentiated into a peripheral zone of periplasm and a single 

 central spherical oosphere. The antheridium is clavate, and 

 applies itself to the side of the oogonium. A fertilization tube 

 enters the oogonium, reaches the oosphere, and ruptures, intro- 

 ducing one or more male nuclei. A true fertilization occurs, and 

 a single oospore is matured. Germination of the oospore is by 

 swarmspores. 



The limits of the family are sharply defined, there being no 

 close relatives in neighboring groups. The development of the 

 sporangia in chains in a definite sorus beneath the surface of 

 the host furnishes a striking point of difference between this 

 group and the two other families of the order. While more 

 highly developed in certain respects than the Pythiaceae it is 



