DIV. I 



THALLOPHYTA 



429 



is produced. They are usually multinucleate, and thus are morpho- 

 logically comparable to a whole gametangium of an isogamous Alga. 

 In the Archimycetes sexual organs have been observed in a few 

 cases only in the form of antheridia and oogonia or of equivalent 

 gametes. 



Multinucleate gametangia, oogonia, and antheridia, which fuse 

 directly with one another, without the separation and escape of the 

 individual gametes, are generally termed COENOGAMETES. 



Order 1. Archimycetes ( 63 ) 



The Chytridiaceae which belong here are microscopically small Fungi parasitic 

 on aquatic or land plants and in some cases on animals. The non-septate mycelium 

 is feebly developed, and is 

 frequently reduced to a single 

 sac -shaped cell inhabiting a 

 cell of the host. Asexual 

 multiplication is effected by 

 means of swarm -spores pro- 

 vided with one or two cilia 

 which enter the cells of the 

 host plant and at first have 

 no cell wall. A cell wall is 

 then formed and the parasite 

 becomes transformed into a 

 multinucleate sporangium 

 which liberates numerous uni- 



nucleate swarm -spores by 



,. i 1 vi FIG. 369. Olpidium Brassicae. A, Three zoosporangia, the 



means of a beak - like pro- contents Qne of which has escaped (x 160) ^ Zoo . 



jection. Thick -walled spor- spores (x 520). C, Resting sporangia (x 520). (After 

 angia which only produce WORONIN.) 

 swarm -spores after a period 



of rest are also developed. The life.- history of Olpidium Brassicae (Fig. 369), 

 which lives as a parasite in the stems of young Cabbage plants and causes their 

 death, is of this type. Synchytrium (Ghrysophlydis] endobioticum, the cause of 

 the wart disease of the Potato, has recently become widespread in Europe. It 

 gives rise to warty outgrowths on the stems and tubers ; these later break down 

 and decay. In summer it forms from the protoplast a sorus of 2-5 spherical, 

 thin-walled sporangia without beak-like projections, and also especially for the 

 winter rest, thick- walled, yellow, resting sporangia. These are at first uninucleate, 

 but on germinating in the moist soil form numerous uniciliate swarm -spores 

 which can enter the cells of the Potato. 



Sexual reproduction as a preliminary to the formation of resting sporangia is 

 only known with certainty for a few forms. In Olpidium Viciae, which is 

 parasitic on Vicia unijuga, the uniciliate swarm-spores are in part asexual, pro- 

 ducing zoosporangia again a few days after entering a cell of the host. In part, 

 however, they behave as planogametes and conjugate in pairs to form naked 

 zygotes provided with two cilia. The zygote settles on the host plant, surrounds 

 itself with a cell Avail and passes its protoplast into the epidermal cell. Within 

 this the zygote develops into a resting sporangium, in which the delayed fusion of 

 the sexual nuclei takes place ; from this, numerous zoospores are developed. 



