Instant Notes: Plant Biology

Thallose liverwortshave a simple thallus (Fig. 1) several cells thick that

branches dichotomously. Some consist of little more than this but, in the more

complex forms, there is a thickening of the central part into a midrib, pores in

the upper surface and rhizoids anchoring them to the substrate. Some species

produce gemmae. Most liverworts contain oil bodies in their cells and some-

times these are fragrant when the plant is crushed, e.g. Conocephalum. They also

produce antibacterial products but the potential of these for human use has not

been examined.

The gametophyte of the hornworts resembles that of a simple thallose liverwort,

with no midrib and with no dichotomous branching. They differ in having cavi-

ties, often with a symbiotic nitrogen-fixing cyanobacterium in them, and, in

most, by having cells containing a single chloroplast, a feature otherwise

unknown in land plants but occurring in the algae. They are separated by differ-

ences in the sporophyte.

The mosses are the most abundant and important of the three groups and a few

grow to 1 m above the ground. After germination, a spore produces filaments of

undifferentiated cells known as the protonema, resembling filamentous algae,

which spread over the substrate. These normally grow for only a few days in

this way before growing into the main gametophyte, though they persist in a

few species. Mosses have simple stems with no specialized cells. Most are

branched and the shoots may grow upright often forming a tight clump or mat,

or trail along the substrate. Rhizoids grow into the substrate, anchoring the

plant but with no special adaptations for absorption, and some mosses grow

similar hair-like structures along their stems. Mosses normally have spirally

arranged leaves, tapered, often to a point, so in general not closely resembling

those of leafy liverworts (Fig. 2). One genus, Fissidens, has two ranks of leaves. A

few mosses produce gemmae.

The majority of mosses have leaves one cell thick (Fig. 2) but there is

frequently a line of narrow thick-walled cells forming a midrib or nerve and

some have another line around the edge. The nerve can extend beyond the end

of the lamina as a hair point. Polytrichumand its relatives have lamellae of cells

along the leaf (Fig. 2), making the leaves fully opaque and much tougher than

Vegetative
structure of
mosses

Vegetative
structure of
hornworts

266 Section P – Plant diversity

(c)

(b)

(a)

50 μm

Hyaline cell

Photosynthetic cell

Fig. 2. Leaves of mosses. (a) Mnium,showing midrib and border; (b) cross-section of Polytrichumleaf showing
lamellae; (c) leaf cells of Sphagnum,showing photosynthetic cells and hyaline cells.