Cell Growth and the Structure and Mechanical Properties of the Wall in Internodal Cells of Nitella opaca: III. SPIRAL GROWTH AND CELL WALL STRUCTURE

The internodal cells of the alga Nitella opaca L, which arein the form of long thin cylinders, exhibit the phenomenon ofspiral growth, i.e. as the cells elongate they also twist abouttheir longitudinal axis. It has been shown in an earlier paper(Probine and Preston, 1962) that the cell wall is mechanicallyanisotropic. In this paper the moduli necessary to describethe elastic behaviour of a material possessing this sort ofsymmetry are considered. It is pointed out that if the Nitellacell is regarded as a thin-walled cylinder built of a materialpossessing orthorhombic elastic symmetry, then there can bea coupling between shear and extension which will produce atorsional twist as the cylinder is pressurized. It is suggested that this is the basic mechanism of spiral growth.Experimental evidence is presented which supports this view.     

Cell Growth and the Structure and Mechanical Properties of the Wall in Internodal Cells of Nitella opaca: II. MECHANICAL PROPERTIES OF THE WALLS

The internodal cells of Nitella opaca L. have been used in anattempt to assess the part which mechanical properties of thewall may play in the control of cell growth. It is shown thatthe wall is mechanically anisotropic in both its plastic andelastic properties, and evidence is presented which indicatesthat this arises from its anisotropy of structure. The degreeof anisotropy is greater in cells with a high growth-rate thanin those with a low growth-rate. Evidence is presented thatthis variation in properties with growth-rate is due wholly,or in part, to changes in the orientation of the crystallinecomponent, in the relative proportion of wall constituents,and in the condition of active groups of the wall components.The findings are in harmony with the theory that extension growthof the cell wall is due to ‘creep’, i.e. disturbancesof the molecular forces within the wall leading to a slow plasticyielding to turgor pressure.     

Cell Growth and the Structure and Mechanical Properties of the Wall in Internodal Cells of Nitella opaca: I. WALL STRUCTURE AND GROWTH

This is the first of two papers dealing with the relationshipbetween growth and the mechanical properties of the wall ininternodal cells of Nitella opaca L. The submicroscopic structure of the cell wall of this alga,as determined by chemical analysis, X-ray crystallography, polarizingmicroscopy, electron microscopy, swelling measurements, andinfra-red spectrography, is described in detail and the changesduring growth are recorded. It has been found that the wallcontains cellulose in the form of cellulose I (type B). Theconstituent microfibrils are preferentially oriented, usuallyin slow helices with considerable angular dispersion about thecommon direction. They are arranged in discrete layers withpectic substances providing an amorphous matrix between microfibrillar-reinforcedlaminations. It is shown that, as the cell elongates, both thestreaming direction in the cell and the mean microfibrillarorientation in the wall change in such a way as to allow thepossibility of a causal connexion between streaming and microfibrillarorientation in a new wall lamella. The orientation in such alamella is undoubtedly modified by subsequent passive extensionmuch as implied in the multi-net growth hypothesis of Roelofsen.     

The Existence of Transcellular Strands in Mature Sieve Elements

Light microscope observations provide further evidence of transcellularstrands in sieve elements. Specimens were prepared by a newmethod for stripping sieve tubes from phloem tissue after fixationin aldehyde solutions. Straight-sided membranous structuresappear in sieve elements and are interpreted as the boundarymembranes of transcellular strands. Occasionally it is possibleto follow these structures through a sieve plate. The proposal that transcellular strands are artefacts due todiffraction lines is discussed and, in relation to present results,is proved incorrect. In the interference microscope, colourcontrast is demonstrated between a surrounding matrix and thestrands at a sieve plate; since this colour is a function ofdensity and thickness in the specimen, structures which displaya particular colour must be real. Spherical granules, some of which are more refractive than others,and fibrils, which may occur in parallel groups to form fibrillarstrands, are sometimes seen within straight-sided boundaries,but more often these constituents occur in the lumina of sieveelements. Fibrils may spread out to fill the whole of the sieveelement and fan-like arrays of fibrils are seen issuing fromstrands in sieve elements and phloem exudate. Membranous and fibrillar strands are orientated parallel tothe long axis in fixed, unstained sieve elements. Subsequentstaining reveals an additional constituent in the same sieveelements which is like a typical slime plug and is distinguishablefrom strands by a lack of structure and heavier staining. Autoradiographs of phloem exudate show an association betweenthe distribution of mobile carbon-14 and parts of transcellularstrands. Mobile carbon-14 within strands appears to be orientatedin parallel lines but there is no evidence of a particulatedistribution of the isotope. This result suggests the mobilecarbon-14 followed the distribution of the parallel fibrilsand is not associated with plastids or granules. A pattern of displaced contents in sieve elements, which isrepeated in superposed cells, indicates the presence of resistanceto pressure change along intact sieve tubes. Such resistancewould prevent movement of solution in response to a pressuregradient and is consequently incompatible with the mass-flowtheory. This result can be explained if the movement of vacuolarfluid through sieve pores is blocked by cytoplasm which introducesa resistance to pressure change between cells along a sievetube.