Phylum Phaeophyta [ 79 



female individuals. Parthenogenesis (reproduction by eggs which have not been 

 fertilized) is rather common in this group. There are no swimming sperms: nuclei 

 from the antheridia reach the eggs through fertilization tubes, or by migration through 

 the periplasm. 



Ziegler found that the first nuclear divisions of the nucleus of the zygote are 

 meiotic: all cells except the zygotes are haploid. 



The organs of asexual reproduction are cylindrical sporangia terminal on the fila- 

 ments. Within these the multinucleate protoplasts undergo cleavage into minute 

 uninucleate spores. It is chiefly by details of the behavior of the sporangia and spores 

 (the latter diplanetic, monoplanetic, or not swimming at all) that the dozen genera 

 are distinguished. Diplanetism is the character of zoospores which are not directly 

 infective; they undergo encystment, and the cysts release infective zoospores. During 

 the first stage of swimming, the spores are pear-shaped, with the nucleus drawn out 

 into a beak toward the narrow anterior end, where the flagella are attached. Spores re- 

 leased from cysts for a second period of swimming are bean-shaped, with the flagella 

 attached laterally, each connected through a separate rhizoplast to the nucleus, which 

 lies at some distance from the cell membrane (Cotner, 1930). No explanation of 

 this behavior, whether by phylogeny, genetics, physiology, or competitive advantage, 

 is known. The apparent trend of evolution is to eliminate it. Monoplanetic spores 

 in the present group are usually released from the sporangia as naked protoplasts 

 which undergo encystment and emerge subsequently as flagellate spores of the second 

 form. 



Saprolegnia releases diplanetic spores through circular pores in the tips of sporangia 

 in which the spores are formed in several rows; new sporangia develop within empty 

 old ones. Organisms which differ from Saprolegnia only in producing new sporangia 

 beside, instead of within, the old ones, were formerly assigned to Achlya, but are now 

 called Isoachlya. Leptolegnia differs from Saprolegnia and Isoachlya in forming 

 spores in a single row. In Achlya proper, the spores are discharged without flagella, 

 to encyst and swim only once. In Thraustotheca the monoplanetic spores are re- 

 leased by irregular breakdown of the distal part of the sporangium. In Dictyuchus 

 the spores become encysted before discharge; their protoplasts escape in the form of 

 secondary swarmers through individual pores in the wall of the sporangium. Salvin 

 (1942) found that cultures while growing release into the medium substances which 

 affect the type of sporangium produced, so that a given culture may be while young 

 of the character of Achlya, and later of the character of Thraustotheca or Dictyuchus. 



Family 2. Leptomitea [Leptomiteae] Kiitzing Phyc. Gen. 150 (1843). Family 

 Leptomitaceae Schroter in Engler and Prantl Nat. Pflanzenfam. I Tail, Abt. 1 : 101 

 (1893). Oomycetes consisting of filaments which are constricted at intervals, but 

 are not differentiated into a basal cell and reproductive branches. In sewage or on 

 organic matter decaying in water. Leptomitus, Apodachlya, Apodachlyella, with 

 some seven known species. The numbers of species and degree of distinction of this 

 family and the following do not appear to justify the proposed establishment of a 

 separate order for them. 



Family 3. Rhipidiacea [Rhipidiaceae] Sparrow in Mycologia 34: 116 (1942). 

 Saprophytes resembling the Leptomitea, the body differentiated into a main part, the 

 basal cell, rhizoids of limited growth, and slender branches bearing the reproductive 

 structures. Sapromyces, Araiospora, Rhipidium, Mindeniella, with perhaps a dozen 

 known species. 



