INTRODUCTION 



8 



spores, and these are Boon transformed directly into 

 ■oospores, without going through a dormant period. 

 In the polysporangiate species of Hoaellopsia 1 1 » » - 

 segments "I the plasmodium form either zoospo 

 rangia or resting spores, but unlike in Woronina 

 they become separated by septa which the host cell 

 forms. As a result, neither sporangia nor resting 

 spores are united in sporangio- and cystosori, re- 

 spectively. In the monosporangiate species of tins 

 genus ele.iv age of the thallus apparently does not oc- 

 cur, since each infection is reported to give rise to 

 one (oosporangium or resting spore. In .ill species 

 nt the Woroninaceae, however, the zoospores encyst 

 temporarily on the surface of the host and form dur- 

 ing germination a penetration tube through which 



the Spore content passes into the host cell as a naked 



protoplast. Nevertheless, marked differences in type 

 of development occur in this provisional family 



which clearly indicate that the generic and family 

 concepts arc inadequate. The resting spores in all 

 three genera appear to he asexual, since no fusion 

 of gametes has been observed during their forma- 

 tion. 



In the second family. Ectrogellaceae. no plasmo- 

 dium occurs, and each zoospore gives rise to one 

 (oosporangium or resting spore. The zoospores of 

 this family, however, exhibit true diplanetism like 

 those of Achilla and other genera of the Saproleg- 

 niales. Resting spores are known in only two spe- 

 cies of this family. In K. perforans they are asexual 

 or possibly parthenogenetie. while in /•.'. Licomo- 

 phorar they have been reported to be zygotic by 

 Scherffel i '25 ). hut the evidence of sexuality in this 

 species is not conclusive. The family Olpidiopsid- 

 aceae includes approximately thirty species which 

 have the same type of vegetative development as 

 those of the Ectrogellaceae. In some species of 

 Olpidiopsis the zoospores exhibit what has been de- 

 scribed as partial and primitive diplanetism, while 

 in 0. Oedogon'iOTum and Pythiella vernalis they are 

 typically diplanetic. Well defined sexuality occurs 



in a large number of members of this family. At the 



close of the vegetative period in Olpidiopsis fusion 

 between thalli of unequal, and rarely of equal, size 

 occurs which results in the formation of a thick- 

 walled zygote. These thalli are generally described 



as OOgOnia and antheridia on the basis of relative 

 size, but they are not markedly differentiated as 



gametangia or gametes. Nonetheless, the evolution 

 and differentiation of such gametangia and heterog 



amy are foreshadowed in Olpidiopsis. The degree 



of sexuality varies considerably in this genus. Some 

 species are wholly sexual or parthenogenetie while 

 others are partially so and form only a few zygotes. 



In Pythiella an egg cell or oosphere with a trace of 

 periplasm is formed in the oogonium— apparently 

 an advance in heterogamy toward the Pytkium type. 



So far as is now known, the development of the thal- 

 lus in the Sirolpidiaceae is similar to that of the 

 Ectrogellaceae and Oljiidiopsidaee.ie with the ex- 

 ception that it may become more elongate and fila- 

 mentous and undergo segmentation. The segments 



thus formed may separate and are transformed di- 

 rectly into olpidioid zoosporangia. The occurrence 

 of resting spores is unknown or at least very doubt- 

 ful in this family, and no evidence of sexuality has 

 been reported. 



In the family I.agcnidiaceae the content of the 

 zoospore does not enter the host as a naked ainoe 

 boid body, but the tip of the germ tube elongates, 

 enlarges, and eventually develops into the mature 

 thallus. In tin- majority of species the thallus is 

 elongate, septate, and may become distinctly my - 

 eclioid. Reduced unicellular, olpidiod thalli. how- 

 ever, are not uncommon. In some species the seg- 

 ments of the thallus may separate as in the Sirolpi- 

 diaceae. The segments are transformed either into 

 zoosporangia or gametangia and both types of re- 

 productive structures may be intermingled in the 

 same thallus. The zoospores may be fully developed 

 in the zoosporangia and swim directly away after 

 emerging or are only partly formed in sporangia and 



undergo further development in an extramatrical 

 vesicle. In other species the content of the sporan- 

 gium emerges to the outside as a naked protoplasmic 

 mass and undergoes cleavage into zoospores in much 

 the same manner as in Pythium. The presence of a 

 vesicular membrane around the protoplasmic mass 

 and the zoospores which are subsequently formed 

 has been reported in a number of species hut appears 

 to be lacking in others. In some species the zoospores 

 encyst in a mass at the mouth of the exit tube and 

 exhibit marked diplanetism. Sexual reproduction in 

 this family is predominantly heterogamous. but in 

 Lagenidium sacculoides, Lagena radieicola, a n d 

 Resticularia nodosa it is reported to be isogamous. 

 The segments of elongate thalli as well as entire re- 

 duced unicellular thalli which function as game- 

 tangia are only slightly or not at all differentiated 

 as sexual organs. The female gamctangium or 

 oogonium, however, is usually larger, more vesicu- 

 lar, and frequently barrel-shaped, while tin' 80- 

 called antheridium is usually elongate and tubular. 

 Differentiation of an egg cell and periplasm in the 

 female gamctangium has not been conclusively dem- 

 onstrated, but the ooplasm may contract and aggre 

 gate toward the conjugation tube or pore during 

 plasmogamy. The zygote which results from fusion 

 lies free in the oogonium and is generally referred to 

 in the literature as an oospore. In some species the 

 antheridium is lacking, and the resting spores are 

 funned parthenogenetically. 



BIBLIOGRAPHY : INTRODUCTION 



Braun, A. 1855a. Her K'gl. Preuss. Akad. Wiss. is;,:,: :is:,. 



. 1855b. Abh. K'gl. Akad. Wiss. 1855: 61. 



Cienkowski, I., 1865. Mot. Zeit. 13: 801. 

 Cornu, M. 1H7.». Ann. Sci. Nat. .-> ser. 15: 113. 

 Fischer, A. 1880. Hot. Zeit. :is : 689. 



. 188S.Jahrb. Wiss. Bot. 13: 286. 



. . 1892. Rabenhorst's Krypffl. I. t::i7. 



Lotsy, .1. 1'. mo?. Vortrage iiber Botanische Stammen- 

 geschichte I. Jena. 



