The different symptoms of resistance coded by the VF gene were studied on glasshouse‐grown apple seedlings after inoculation with a conidial suspension of Venturia inaequalis. Scanning‐electron microscopy and histological observations defined precisely three different classes of symptoms. Class 1 corresponds to a hypersensitive reaction. Class 2 is characterized by considerable tissue modifications but no sporulation whatsoever occurs; this reaction defines resistant hybrids. In class 3 the fungus remains active and can sometimes sporulate abundantly; this class was divided into two subgroups, one comprising all the hybrids of weak resistance, the other the hybrids of weak susceptibility.

The synthesis in vitro of metachromatic granules was demonstrated in the mycelia of Pa.xillus involutus (Batsch) Fr. both in pure culture and associated with hirch {Betula pendula Roth) roots. X-ray microanalysis revealed the presence of phosphorus and calcium in the granules of mycorrhizas. Pliosphorus-3 1 NMR spectroscopy was used to identify intraeellular orthophosphate and polyphosphates. In the myeelia, orthophosphate represented the major fortn of phosphorus accumulation, and a small pool of polyphosphate was detected in the oldest phases of culture. On the other hand, an important NMR resonance corresponding to polyphosphates was identified in the spectra of synthetic mycorrhizas. The association between a mycelium and a root seems to modify the metaholism of fungus considerately.

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Thesium humile Vahl (Santalaceae) is a root-hemiparasite which causes economic losses in cereal crops in the Mediterranean countries. O2 exchange measurements showed that photosynthesis does occur in Thesium. Experiments using 14CO2 gave additional evidence of the photosynthetic capacity of T. humile at both preparasitic and parasitic stages; roots of T. humile, unlike those of non-parasitic plants, exhibited a low sink strength for photosynthates. The end product of photosynthesis in T. humile is mannitol which is also the main carbohydrate translocated in the phloem. Nevertheless, feeding host leaf with [3H]glucose clearly demonstrated that the parasite, in spite of its photosynthetic capacity, remained dependent upon its host for an additional supply of reduced carbon. Moreover, in isolated T. humile shoots supplied with [14C]sucrose via the transpiration stream, the absorbed sucrose was rapidly converted into mannitol. Thus the parasite would be able to convert sucrose derived from the host plant into mannitol. The present work gives clear evidence that mannitol can be regarded as the major soluble carbohydrate in the carbon metabolism and in the translocation of reduced carbon in T. humile. The mannitol synthesis pathway might be a metabolic target for the chemical control of this parasite in cereals, in which this polyol has never been detected.