The medullary rays extend radially from the centre of the tree to the bark at right angles to the grain of the wood, and serve during life to bind the whole together as well as to convey nourishment from one part of the tree to another.
The coenenchyma is further differentiated into a medullary portion and a cortex.
The pericycle, medullary rays, endocycle and mesoderm all form parts of one tissue system, the external conjunctive, and are only topographically separable.
Among Gymnosperms the secondary xylem is similarly simple, consisting of tracheids which act as stereom as well as hydrom, and a little amylom; while the phloem-parenchyma sometimes undergoes a differentiation, part being developed as amylom, part as proteid cells immediately associated with the sieve-tube, in other cases the proteid cells of the secondary phloem do not form part of the phloem-parenchyma, but occupy the top and bottom cellrows of the medullary rays, the middle rows consisting of ordinary starchy cells.
Insigne, has scalariform tracheides, and is traversed by regular medullary rays, but in the forms from later ' horizons the tracheides are reticulately pitted, and the rays are for the most part replaced by a network of xylem-parenchyma.
An important feature is the occurrence in some species (Ptychoderidae) of paired longitudinal pleural or lateral folds of the body which are mobile, and can be approximated at their free edges so as to close in the dorsal surface, embracing both the median dorsal nerve-tract and the branchial grooves with the gill-pores, so as to form a temporary peri-branchial and medullary tube, open behind where the folds cease.
The xylem and phloem parenchyma consist of living cells, fundamentally similar in most respects to the medullary ray cells, which sometimes replace them altogether.
Abnormal female of the medullary rays in Flowers.
These collateral bundles are separated from one another by bands of conjunctive tissues called primary medullary rays, which may be quite narrow or of considerable width.
The typical heteromerous thallus shows on section a peripheral, thin and therefore transparent, layer, the cortical layer, and centrally a mass of denser tissue the so-called medullary layer, between these two layers is the algal zone or gonidial layer (figs.