Page 151 - Biology_F5

Biology for Advanced Level Secondary Schools

mountain forests of Tanzania, such as the or petiole. Characteristically, the young
Udzungwa scarp nature forest reserve. This leaves show a tightly rolled structure
fern can grow to a height of about 5 metres. called circinate leaves or croziers which
Ferns have conducting tissues (xylem and later unroll to reveal fronds with a stalk or
phloem) that are not well developed and petiole at the base. The bases of the fronds
are sometimes termed as tracheophytes. are covered with dry brown scales called
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While most pteridophytes are homosporous, ramenta for protection of young leaves
some of the fern species are heterosporous against drought. The frond has a midrib
produce two types of spores. An example called rachis, which bear leaflets called
of a common fern is Dryopteris filix-mas, pinnae on both sides and depending on
found in damp woods and other shady species small rounded sub-divisions of
places. Besides the pteridophytes, which are pinnae called pinnules (Figure 3.24).
regarded as true ferns, other plant species
called fern allies are not true ferns, though The mature frond bears specialised
they relatively resemble pteridophytes in reproductive structures called sori (clusters
various ways. For example, they disperse of sporangia) on the lower surface of the
by shedding spores to initiate an alternation frond. The sporangia contain spores;
of generation. therefore, sporangia bearing leaves are
called sporophyll (sporo meaning spore
Structure of Dryopteris sp. and phyll meaning leaves). The Dryopteris
A mature fern plant has an underground species have poorly developed vascular
creeping stem called rhizome. This bears tissues with simple xylem elements for
adventitious true roots for anchorage and the mechanical support and transport of
absorption of water and mineral salts from water and mineral salts, while phloem
the soil. It has broad leaves called fronds elements are mainly for the translocation
attached to the rhizome by a long stalk of synthesised food.

Leaflet (Pinna)

Sorus

Fronds
Rachis

Crozier

Young leaf Petiole (stalk)
Rhizome
Adventitious roots

Figure 3.24: Structure of Dryopteris sp. showing sori at the lower side of the frond

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