Internal Pressure of Hyphal Tips of Fungi, and its Significance in Morphogenesis

Some simple indications of the presence of pressure in hyphaltips have been used to investigate osmotic relationships. Theseare (1) cytoplasmic flow due to pressure loss through the apexcaused by janus green solutions, (2) extrusion of cell contentson damaging the cell wall with dilute acetic acid, and (3) continuationof apical extension growth. The osmotic equivalent of growingtips is considerably in excess of that of the growth medium,resulting in the presence of a hydrostatic pressure differentialbetween the interior of the tip and its environment. Normalextension growth is dependent upon this differential. Hyphaltips are able rapidly to equilibrate osmotically to appliedhyper- and hypotonic solutions so as to re-establish a pressuredifferential consistent with the rate of synthesis of wall materialat the apex. Apices that are caused to cease extension growthby applied hypertonic solutions enter upon a sequence of slightbut significant morphological changes leading to progressiveocclusion of the apex by continued formation of wall substancein the absence of a sufficiently high pressure differential.The subsequent behaviour of such apices depends upon the stagein this sequence reached at the re-establishment of a pressuredifferential sufficient for extension growth.