Zoosporangia of Polymyxa betae Keskin are plurilobate and develop cross walls. During ripening they show dark zones at the contact points with the host cell wall at the light microscope level. With the electron microscope these zones appear to correspond to single uninucleate compartments separated by a wall from the zoosporangium itself. These compartments give rise to exit tubes while the zoosporangial contents differentiate into zoospores. At the beginning of exit-tube formation, the thallus wall in contact with the host cell wall becomes thinner and softened. At the same time, thallus cytoplasm shows membrane-bound vesicles, the endoplasmic reticulum increases, and vacuoles enlarge. Vesicles are concentrated in the area of exit-tube formation and this suggests that they are involved both in host and thallus wall lysis. Wall microfibril profiles indicate that at this point mechanical forces have also been applied. A papilla, surrounded by a wall which appears to be continuous with the inner layer of the zoosporangium wall, is formed. The papilla elongates and becomes tubular, forming the zoospore exit tube. The same lytic and mechanical sequence described for thallus and host cell wall demolition at the site of exit-tube emission, also takes place in the demolition of zoosporangial cross walls.

Parasitism of Trichoderma harzianum on Cystosori of Polymyxa betae Transmission electron microscope investigations revealed that cystosori of Polymyxa betae Keskin are, in vitro conditions, parasitized and completely degraded by Trichoderma harzianum Rifai. The hyperparasite showed a high ability in colonizing excised, surface‐sterilized lateral roots of sugar beet and in invading the cystosori present in them. Both the walls of the root cells and of the cysts are enzymatically perforated. However, in cysts penetration, mechanical forces are also involved. After penetration, cysts undergo a rapid degradation and finally they appear to be completely empty or to contain only small amounts of degradated cytoplasm. Cyst walls are also strongly altered and deformed and only two thin osmiophylic residual layers are detectable.

Cerutocystis fimbriata (Ell. et Halst.) Davidson /. pLitani Walter development in Plane-tree {Platanus orientalis L.) tissues was examined by seanning eleetron mieroscope. All the fungal elements are smooth exeept for doliform endoconidia which are diaracterized by a rough surface and protruding rims.The fungus penetrates lesions and spreads rapidly along medullary rays into the pith and finally into the xylem elements. The hyphal presence in pith cells causes the disappearance of plastids and breakdown of the middle lamellae. The internal walls of vessels are not affected by hyphal activity during the period of the study.

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