Hyphomycetous fungi are either in fact, or by supposition, imperfect forms of the Ascomycetes and Basidiomycetes. Their 'spores ' may be formed by simple fragmentation of the hyphae or, in a variety of more complex ways, by budding or by specialized growth at the hyphal apex. Thesespore-bearing parts are in turn borne as simple branches on the prostrate hyphae, or as terminal branches of an erect and often very beautiful branch system.These branch systems, although characteristic for the taxonomist, are to some degree plastic with changing environment. It is well known to mycologists that fungi such as Botrytis cinerea Pers. and Penicilliurn spp. will fail to spore, or produce atypical conidiophores with few spores when grown in % saturated atmosphere ; similarly that variations in the medium, in the incident light and in a variety of other environmental factors, may profoundly influence the shape of the conidiophore. Thus the genetic control of shape of the conidiophore is often closely linked to a particular set of environmental conditions and comparatively small changes in the environment upset the genetic control. The control of shape in unicellular organisms poses problems which differ a t least in degree from those in multicellular organisms with multicellular regions of differentiation. The Hyphomycetes which consist essentially of a string of cells left behind by a single apical cell pose problems which are similar to the problems in unicellular organisms for detailed development, but resemble those in more complex plants and animals in thrtt there is an overall control of conidiophore shape. At first sight an analysis of the processes involved in the differentiation of the branch system and of the ultimate sporebearing parts is too complex for present methods, although clues might be sought in changes in shape with changes in environment, but, if they have been, no general explanation of the mechanism of conidiophore differentiation has been given.Another approach has been possible following observations on the behaviour of the hyphal apices of Fusarium oxysporurn Schl. immersed in solutions of differing osmotic pressure. It was shown (Robertson, 1958) that the solutions used led to changes in the behaviour and appearance of the main growing hyphal apices on the agar plate. These changes may be summarized as in Table 1.The explanation offered is that an alteration in the rate at which water enters the hyphal apex affects the whole mechanism of extension and may bring about arrestment. I n an experiment in which the hyphae were immersed in water for from 5 sec. to 20 min. before being transferred to an isotonic solution it was clear that exposure of apices to water for 10 sec. was suEcient to bring about arrestment and the subsequent swelling and branching. What was more surprising was that when the fungus was exposed to water for 40 sec. or less, before being transferred to an isotonic solution, all apices swelled and branched, whereas exposure to water for more than 40 sec. before transfer t o the isoto...

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