The commonest species is P. fastigiata, which may be found in abundance as bushy brownish tufts on the fronds of Fucus nodosus (p. 386). A transverse section of this weed is a very beautiful microscopic object. It resembles a wheel, with a dark centre to the nave, and several spokes enclosing about sixteen regularly arranged tubes. The swollen tips of fronds should also be examined for the urn-shaped cells containing the spores; and if a gentle pressure be applied to the cover-glass with a needle, the little pear-shaped spores may be expelled. The other kind of spores may be found near the bases of the branches on different plants.
Fig. 256.—Polysiphonia fastigiata
Among other species we may briefly mention—P. parasitica, sometimes found near low-water mark, growing in little feathery tufts of a bright-red colour, on the lichen-like Melobesia or on corallines. It has seven or eight parallel siphons in its fronds, all regularly arranged round a small central space.
P. Brodiæi is moderately common on our coasts. This is a large brown species, with seven siphons surrounded by a thick cellular layer which conceals the articulations and is too opaque to allow the siphons to be seen without dissection. Its branches, which are alternate, bear short tufts of delicate branchlets.
Fig. 257.—Polysiphonia parasitica