SPIRAL GROWTH AND REVERSAL OF SPIRALING IN PHYCOMYCES, AND THEIR BEARING ON PRIMARY WALL STRUCTURE

MOST INVESTIGATORS would agree that there is a high degree of orientation in the structure of the plant cell wall. In some measure this extends even to the structure of the tenuous, growing, primary wall of the enlarging cell. Such structural orientation is shown largely by the occurrence of cellulose in the form of microfibrils, themselves crystalline, lying in one or more preferred orientations, usually in the plane of the cell surface. Cell enlargement is accompanied by expansion or growth of the primary wall, and involves the deposition of new wall material in an oriented pattern. What determines the characteristic form of cell growth and the pattern of its wall structure has never been clear. Plant growth hormones probably act not directly on the wall but at some prior stage in the complex process of growth. On the other hand, microscopic and X-ray investigations have been most successful in dealing with secondary wall structures where the cell is already beyond the stage of active enlargement. Thus, although the primary wall is of the greatest physiological interest, we have the least satisfactory knowledge of its fine structure and mode of growth. There is, however, an additional line of evidence that can be brought to bear on primary wall structure. In some types of cell the orientation of the growth process can be studied. Where this has been possible, growth has been found to be highly oriented, and it appears that such growth co-exists with oriented structure of the growing wall. Oriented growth-may be defined as active enlargement along a preferred axis or axes. For example, many cylindrical cells have attained their shape primarilv by elongation, and might as a first approximation be said to possess growth oriented along their long axis. Tissue cells, however, may have been passively oriented in their growth by mechanical influences. Hence, it is not possible to consider from this point of view such interesting cell types as the cambial elements or the much-studied cells of the Avena coleoptile. The occurrence of oriented growth in cells which are at all free to determine their own shapes clearly indicates the existence of a directional property in the structure of the growing wall. Some authors have regarded the preferred axis of cell enlargement as directly determined by the existing wall structure (Bonner, 1935; van Iterson, 1937; Diehl, et al., 1939; Ziegenspek, 1939, 1941). Broadly interpreted, this view should permit inferences as to orientation of structure from studies of oriented primary wall growth. It is regrettable that we lack adequate information about the orientation of growth in cells whose primary wall structure is best known, such as the 1 Received for publication June 18, 1942. cotton fiber (Anderson and Kerr, 1938; Berkley, 1939; Hock, Ramsay, and Harris, 1941). Almost the only available data relate to the growth of the sporangiophore of Phycomyces, where a spirally oriented form of growth occuts. The mature cylindrical sporangiophore elongates by growth which is restricted to an apical zone proximal to the terminal sporangium. The axis along which elongation takes place is not parallel to the long axis of the cell, but makes an angle with it which may be as much as 300. Hence, the sporangium and the terminal region of the cell are rotated about the long axis during growth, and the cylindrical sporangiophore forms a spirally-wound wall structure (Oort, 1931; Oort and Roelofsen, 1932; Castle, 1937, 1938). Spiral growth is by no means confined to Phycomyces. It exists in the related fungus Mucor mucedo,2 and according to Astbury and Preston (1940) occurs in Cladophora where, significantly, spiral wall structure also is found. It is likely that suitable growth studies will reveal that the incidence of spiral growth is more general than has been supposed. Heyn (1939) has listed some five attempted explanations of spiral growth, as many as two "theories" being ascribed to the present writer. It is evident that where there is so much theory there must be a dearth of fact, and the work described in this paper was undertaken primarily to give a more complete picture of the facts of spiral growth in Phycomyces. MATERIAL AND METHODS.-The "plus" strain of Phycomyces blakesleeanus was grown in small vials on autoclaved white flour paste, which supports luxuriant growth. Isolated sporangiophores were obtained by clipping away all but one of those present in a culture. In order that growth might be followed in localized regions, individual markers were attached to the sporangium or to the sporangiophore with a micromanipulator. Single spores of Lycopodium were found to make excellent markers. They were easily attached by a touch of the micromanipulator needle and showed up well in photographic records of growth. In general, automatic photographic recording was employed, using a special camera magnifying ten times. The culture vessel with a suitably marked sporangiophore rested in a moist, glass-walled chamber with a constant dim light overhead to ensure vertical growth. The camera was placed in front of the chamber and was driven parallel to the chamber at a slow, constant rate by a lead screw turned by a synchronous motor acting through a train of gears. Periodically, the sporangiophore was brightly illuminated with red light from one side and automatically photographed, the travel of the camera being briefly in-