EXPLANATION OF TERMS
Fig. 4 A frond is once-pinnate when the incisions extend to the midvein (Fig. 3). Under these conditions the midvein is called the rachis (a), and the divisions are called the pinnæ (b).
A frond is twice-pinnate when the pinnæ are cut into divisions which extend to their midveins (Fig. 4). These divisions of the pinnæ are called pinnules (a).
Fig. 5 A frond that is only once-pinnate may seem at first glance twice-pinnate, as its pinnæ may be so deeply lobed or pinnatifid as to require a close examination to convince us that the lobes come short of the midvein of the pinnæ. In a popular hand-book it is not thought necessary to explain further modifications.
The veins of a fern are free when, branching from the midvein, they do not unite with other veins (Fig. 5).
Ferns produce spores (Fig. 6) instead of seeds. These spores are collected in spore-cases or sporangia (Fig. 7). Usually the sporangia are clustered in dots or lines on the back of a frond or along its margins. These patches of sporangia are called sori or fruit-