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).
Fig. 6
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-
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