Instant Notes: Plant Biology

fossil closely resembling Equisetumitself, so this may be an ancient genus. The

best known fossil Equisetopsids, also from the Carboniferous period, were large

trees forming an important constituent of coal, Calamitesand its relatives.

Calamites(Fig. 5) grew to approximately 20 m in height but had many features

similar to those of living horsetails. They had jointed ribbed stems, extensive

branching and scale-like leaves, though these were often bigger than those of

living horsetails. There was an underground rhizome system from which trunks

grew and it is likely that these trees formed large patches. The stems had a large

central pith but secondary thickening was extensive outside this. Strobili were

borne on the side branches rather than at the tips but varied in structure, some

having leaf-like bracts by each sporangium, and some clearly showing true

heterospory with two different sizes of sporangium. Some species retained the

megaspores in their sporangia on the parent plant to be dispersed as a unit.

Q2 – Clubmosses and horsetails 283

Sporangia

Sporangial branch

Spore

Elater

0.5 mm

100 μm

(a) (b)

(c)

Fig. 4 (a) A branch of the strobilus of a horsetail showing sporangia; (b, c) a spore with attached elaters in dry (b) and
moist (c) conditions.

1 m

Fig. 5. Reconstruction of Carboniferous member of the Equisetopsida, Calamites.