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Fungal hyphae characteristically extend at the apex, by the localized deposition of new cell wall and plasma membrane. That entails the performance of work and raises the question, what forces drive hyphal extension in the face of surface cohesion and external resistance? Conventional wisdom credits turgor, i.e., hydrostatic pressure, with driving the tip forward and shaping it by plastic deformation. An experimental test of this hypothesis became possible with the discovery that the oomycetes Achlya bisexualis and Saprolegnia ferax do not regulate turgor. When the osmotic pressure of the medium is raised by the addition of sucrose or other osmolytes, the organisms produce a more plastic wall and continue to grow. Saprolegnia ferax produces near-normal hyphae in the absence of any measurable turgor. Responses to variations in the composition of the medium and to a range of inhibitors indicate that the processes responsible for growth are the same in normal hyphae (4 bars; 1 bar = 100 kPa) and in turgorless ones. Our observations imply that hyphal extension in oomycetes has much in common with pseudopod extension in animal cells, in that polymerization of the actin meshwork in the apical region plays an indispensable role. In the extreme case, when turgor is essentially zero and the wall is most plastic, actin polymerization may contribute substantially to the driving force for extension. But when turgor is high and the wall rigid, hydrostatic pressure is likely to be required to stress the wall, allowing it to expand and admit new wall material.RCsumC : Les hyphes fongiques s'accroissent typiquement par leur apex, par la deposition IocalisCe de nouvelles parois cellulaires et d e plasmalemme. Ceci necessite la realisation d'un travail et souleve la question a savoir quels sont les f o r~e s qui conduisent 8 l'extension des hyphes contre la cohesion de surface et la resistance externe. Le sens commun fait intervenir la turgescence, c'est-8-dire, la pression hydrostatique, qui projetterait I'extrCmitC vers l'avant, tout en le formant par deformation plastique. Une verification experimentale de cette hypothkse est devenue possible avec la decouverte que les oomychtes Achlya bisexualis et Saprolegnia ferax ne regularisent pas leur turgescence. Lorsque la pression osmotique du milieu est augmentee par I'addition de sucrose ou autres osmolytes, ces organismes produisent une paroi cellulaire plus plastique et continuent 8 croitre. Le S. ferax produit des hyphes presque normaux en absence de toute turgescence decelable. Les reactions aux variations de la composition du milieu et 8 un ensemble d'inhibiteurs, indiquent que les processus responsables de la croissance sont les m&mes dans les hyphes normaux (4 bars; 1 bar = 100 kPa) que dans les hyphes non-turgescents. Les observations effectutes pas les auteurs impliquent que I'extension de l'hyphe, chez les oomycktes, ressemble beaucoup 8 l'extension des pseudopodes chez les cellules animales, en ce que la polymerisation du reseau d'actine de la region apicale joue un rBle ...

We have isolated a remarkable filamentous siderable number of infusions were prepared from marine organism, Leucothrix mucor, which was algal thalli and incubated in diffuse daylight at first described in 1844 by Oersted (1). Leucothrix 25 C. Seven species of algae were used, of which may be characterized succinctly as a chemohet-one belonged to the Chlorophyta, two to the erotrophic counterpart of the colorless sulfur-Phaeophyta, and four to the Rhodophyta. Only oxidizing organism Thiothrix. It has been ob-infusions prepared from the green alga, Ulva served on a few occasions (2, 3) in the century lactuca Linn., gave rise to the development of since its original description, but the existing Leucothrix, which appeared without fail in every accounts of its morphology and development, flask containing Ulva. The enrichment procedure based entirely on the examination of crude culis, of course, far from specific. Within a few days tures, are either incomplete or inaccurate. Thanks a pellicle of bacteria and protozoa forms on the A repetition of the enrichment experiments was In liquid cultures, Leucothrix grows in the form undertaken to discover, if possible, what factor of long, colorless, unbranched, tapering threads determined the appearance of Leucothrix. A con-which are attached basally to the wall of the cul-49

Hyphae of Achlya bisexualis growing on a medium deficient in amino acids elongated but produced relatively few branches. Branching was enhanced by three classes of compound: cytochalasins A and E, the calcium ionophores A23187 and ionomycin and proton ionophores such as tetrachlorosalicylanilide (TCS), carbonylcyanide m-chlorophenylhydrazone (CCCP), and carbonylcyanide p-trifluoromethoxyphenylhydrazone (FCCP). We suggest that the effects of cytochalasins reflect the disruption of a microfilament-based system for vesicle transport. Enhancement of branching by ionophores implicates cytoplasmic ions in the control of branch initiation. There may be links between these phenomena and the earlier discovery that a new point of proton entry precedes the emergence of a branch and predicts its locus.