54 COMPARATIVE MORPHOLOGY OF FUNGI 



The Saprolegniaceae are also aquatic but are generally saprophytic on 

 animal and plant remains; their thallus is well developed. The oogonial 

 contents collect at the wall, surrounding a large central vacuole, and 

 split into several egg cells. The Peronosporaceae are endoparasitic in 

 land plants. No central vacuole is formed in their oogonia, whose 

 contents are differentiated into a vegetative periplasm and a gonoplasm 

 which forms the single egg. 



The morphological relationships between these six families are 

 represented on p. 53. Several parallel developmental series have been 

 distinguished, where Gonapodya, Leptomitus, Apodachlya, Pythiopsis 

 and Achlya take approximately the same level. On account of the slight 

 differentiation of their oogonia and their diplanetism, the Saproleg- 

 niaceae stand at the bottom of the biflagellate series. The Leptomita- 

 ceae show great similarities to the Blastocladiaceae. This may be only 

 a convergence phenomenon, as both groups live in stagnant water on 

 decaying objects where other Oomycetes do not appear. Apparently 

 the Peronosporaceae have arisen from Saprolegnioid forms; the organiza- 

 tion, however, of the sporangia of the present known genera (as the 

 simpler Pythieae) is lower than that of the modern Saprolegniaceae; on 

 the other hand, the Pythieae, in their zoosporangial germination, still 

 bear traces of earlier diplanetism and roughly correspond with Achlya. 

 In this respect they are above Saprolegnia. The Ancylistaceae are 

 connected to the Pythieae; both are partially endosparaltic and similar 

 in their habits (Atkinson, 1909a). As regards their sexual organs, the 

 Ancylistaceae would be better connected to the present Saprolegnia, and 

 below the lowest Pythieae, as their oogonia are more primitive than those 

 of other biflagellate Oomycetes. 



A short discussion of the mutual phylogenetic relationships of the 

 Oomycetes, especially the derivation from the green algae, indicated in 

 the scheme on page 53, will be deferred to the close of the order in con- 

 nection with the Peronosporaceae. 



Monoblepharidaceae. — This family occupies a special position in the 

 fungi, because of the motility of its zygotes, elsewhere known only in 

 Olpidium, and because of fertilization by motile sperms. Monoblepharis 

 is best known. It is found in Europe and North America on fallen twigs 

 in water, especially in spring and fall. The zoospores formed in the 

 spring germinate the following fall, and on twigs which have lain in water 

 all summer, develop a new vegetation whose oospores then winter over 

 and develop further the next spring. In summer they seem suppressed 

 by algae. 



The thallus consists of rhizoids which penetrate the substrate, and of 

 multinucleate, little-branched, extramatrical hyphae, only distinguishable 

 from the otherwise similar but thicker hyphae of Saprolegnia by their 

 faveolate protoplasm. At the beginning of unfavorable growth condi- 



