SummaryMacrozamia communis exhibits root dimorphism, possessing both normal and coralloid roots. The latter are pneumatophores in which an algal zone may be present or absent. In the coralloid roots the root cap tissue was interpreted as forming a secondary cortex which persisted throughout the life of the root. Underlying the secondary cortex was a transformed epidermis, the cells of which, following the establishment of algae within the intercellular spaces, elongated radially to form an algal zone easily visible to the eye. Nostoc-like blue-green algae were isolated from this zone. In the absence of algal infection the transformed epidermis remained inconspicuous. The root cap anatomy of the normal roots was similar to that described for other gymnosperms. The tips of the alga-free coralloid roots were characteristically located near the soil surface. The alga-containing coralloid roots were less strictly negatively geotropic and were found at depths up to 30 cm below the soil surface. INTRODUOTIONThe coralloid roots of Macrozamia communis L. Johnson (syn. M. spiralis Miq.), containing endophytic blue-green algae, were shown by Bergersen, Kennedy, and Wittmann (1965) to fix nitrogen. These negatively geotropic roots have been little studied. Working with M. spiralis, McLuckie (1922) concluded that the coralloid roots represented "tubercles" formed in response to bacterial infection and that no algal zone was present. Later Schaede (1944), who also studied M. spiralis, observed an algal zone but not bacteria. Both authors agreed that lenticels were absent.Since Reinke (1872, quoted by McLuckie 1922) first described the association between blue-green algae and the coralloid roots of Oycas revoluta many papers have dealt with similar structures in other members of the Cycadaceae. This literature is most recently reviewed by Schaede (1944,. Typically the blue-green algae in cycad coralloid roots are found localized in a discrete algal zone, which is enclosed by an outer cortex, absent in normal roots. McLuckie (1922) has theorized that the outer cortex corresponds to a persistent root cap. This theory has not been supported by later workers and Schaede (1944) is emphatic that no root cap is present on the coralloid roots. Goebel (1932), without anatomical examination, has further suggested that the cycad coralloid roots were formed primarily as pneumatophores.The factors responsible for the initiation of coralloid roots are considered uncertain. McLuckie (1922) and Watanabe (1924) have suggested that initiation was induced respectively by bacteria and anaerobic conditions. As algae, fungi, and bacteria were not consistently observed in coralloid roots, Goebel (1932) and Schaede (1944) have tentatively put forward the view that microorganisms were not responsible for coralloid root initiation. SummaryMacrozamia communis exhibits root dimorphism, possessing both normal and coralloid roots. The latter are pneumatophores in which an algal zone may be present or absent. In the coralloid roots the root cap ti...

SUMMARYAnabaena showed a seasonal difference in nitrogen storage when growing in Cycas revoluta coralloid roots. In winter Anabaena had numerous cyanophycin granules while in summer there were few cyanophycin granules and many degenerate algal cells. Cellular protrusions were present in the developing algal zone of coralloid roots which may facilitate metabolic exchange between the host and phycohiont. By ferric chloride ultrahistochemistry, phenolic deposits were found in the mucilaginous material of the algal zone. Phenolic deposits were also found in adjacent cells on the plasma membrane, endoplasmic reticulum, within plastids, and at the periphery of coralloid roots. The distribution of phenolic constituents could therefore provide a mechanism for excluding micro-organisms and permitting only Anabaena to grow in cycad coralloid roots.

SummaryCoralloid roots of Macrozamia communis have been shown by the isotopic method to fix nitrogen when they contain the endophytic blue·green algae. Immature coralloid roots devoid of the endophyte did not fix nitrogen. Coralloid roots from glasshouse·grown plants fixed 2·7 times as much nitrogen when illuminated than they did in the dark and the 15N excess was about equally divided between fractions soluble or insoluble in 3N HCI. Coralloid roots excavated from beneath large fieldgrown plants were opaque and did not fix more nitrogen when illuminated than they did in the dark. Most of the newly fixed nitrogen was found in the buffered sucrose extract of crushed tissue. When an intact plant bearing coralloid roots was exposed to an atmosphere containing a large excess of 15N. for 48 hr the 15N was found to be distributed through the plant parts. Nitrogen fixed in the coralloid roots is thus available for the growth of the plant. The coralloid roots evolved small amounts of hydrogen.

Developing plastids in young tobacco leaves contain thylakoidal bodies, inclusions bound by a single membrane continuous with stroma lamellae. Both the thylakoidal body and its attached lamellae contain an enzyme that catalyzes an oxidation reaction with 3,3'‐diaminobenzidine (DAB). DAB staining of the thylakoidal body and lamellae is not the result of photo‐oxidation and is inhibited by potassium cyanide. The thylakoidal body disappears as plastids develop into chloroplasts and, further, the lamellar systems of the mature chloroplasts do not stain with DAB. In developing chloroplasts, it is suggested that the thylakoidal body forms by accumulation of protein which stains with DAB within primary lamellae derived from the inner plastid membrane. The ultrastructural and cytochemical evidence suggests that the thylakoidal body stores protein used later in lamellar formation.

Developing plastids in young tobacco leaves contain thylakoidal bodies, inclusions bound by a single membrane continuous with stroma lamellae. Both the thylakoidal body and its attached lamellae contain an enzyme that catalyzes an oxidation reaction with 3,3'‐diaminobenzidine (DAB). DAB staining of the thylakoidal body and lamellae is not the result of photo‐oxidation and is inhibited by potassium cyanide. The thylakoidal body disappears as plastids develop into chloroplasts and, further, the lamellar systems of the mature chloroplasts do not stain with DAB. In developing chloroplasts, it is suggested that the thylakoidal body forms by accumulation of protein which stains with DAB within primary lamellae derived from the inner plastid membrane. The ultrastructural and cytochemical evidence suggests that the thylakoidal body stores protein used later in lamellar formation.