Vascular Tissues

The steles—cylinders of vascular tissues in the centres of fern stems—exhibit somewhat diverse patterns. Most common ferns possess a “dictyostele,” consisting of vascular strands interconnected in such a manner that, in any given cross section of stem, several distinct bundles can be observed. These are separated by regions filled with parenchyma cells known as leaf gaps. There are, however, numerous “siphonostelic” ferns, in which the gaps do not overlap and a given section shows only one gap, and some “protostelic” ferns, in which no gaps at all are formed. Complex stelar patterns are known in some species, as in the common bracken fern (Pteridium), which has a polycyclic dictyostele, in which one stele occurs within another stele. Large strands of fibrelike cells running between the two steles form mechanically specialized hard tissue, or sclerenchyma.