330
T.G.A. Green and O.L. Lange
only of the boundary layer resistance. The lack of any other gas-phase
resistance is clearly shown by helox experiments on the thalloid liverwort,
Monoclea forsteri (Cowan et al. 1992). In this work photosynthetic rate is
measured both in air and in helox (21% oxygen, 79% helium). Since CO 2
diffuses 2.3 times faster in helox, an increased A is obtained at any chosen
CO2 concentration, in he lox compared to in air if a gas-phase diffusion
resistance is present and CO2 is limiting photosynthesis. In M. forsteri, A is
identical in both air and helox indicating the absence of any significant gasphase resistance. Some form of cuticle (resistance re in Fig. 16.2) is now
known to exist, even on leafy mosses, and any increase in cuticular resistance
to water loss will also affect CO2 uptake (Proctor 1981). Most of the
resistance to CO2 diffusion is between the exchange surface and the photosynthetic centers (Nobel 1977). The presence of these large, non-gas-phase
resistances is suggested by the response of A in bryophytes to CO2 concentration. Typically A does not saturate until well above 1000 III CO2 1-1 air
(see Sect. 16.2.6). One value cited for the CO2 diffusion resistance is
95 scm- 1 (Mnium ciliare, Nobel 1977).
The boundary layer resistance can be increased in leafy liverworts and
mosses by arrangement of shoots into clumps or turfs, or by terminal,
nonphotosynthetic structures such as hair points (Proctor 1981). Moreover,
the photosynthetic structures, "leaves", can be arranged in a form of microcanopy that can achieve the following high leaf area indices: 1, Monoclea
forsteri, thalloid liverwort (Green and Snelgar 1982a); 6, Tortula intermedia;
18, Mnium hornum; 20-25, Pseudoscleropodium purum (Proctor 1979);
15, Drummordia prorepens (Vitt 1990); 44, Tortula ruralis; 129, Ceratodon purpureus (Simon 1987). Entire microcanopies would be expected to
achieve higher photosynthetic rates than individual stems through better
light utilization.
b) Ventilated structures, photosynthetic tissue not at the surface (Fig.
16.2).
1. behind a pore: thalloid liverworts (Marchantiales),
2. behind a slit: members of the Polytrichales (parallel photosynthetic lamellae on the upper leaf surface have enlarged, wax-covered terminal cells
that confine gas-exchange to a fine slit between the lamellae).
This grouping contains the endohydric mosses and liverworts, plants that
have variously water-proofed surfaces, often particularly well developed
near gas-exchange pores (SchOnherr and Ziegler 1976; Clayton-Greene
et al. 1985), and a significant internal water-transport pathway. Although
poikilohydric, they are, in many ways, equivalent to homoiohydric higher
plants with "leaves" having an increased internal area to surface area ratio,
for instance 9 for Marchantia foliacea (Green and Snelgar 1982a). According
to Nobel (1977), a higher internal area to surface area should allow higher A
and certainly some of the higher values for A in mosses are those of
Polytrichum species.
T.G.A. Green and O.L. Lange
only of the boundary layer resistance. The lack of any other gas-phase
resistance is clearly shown by helox experiments on the thalloid liverwort,
Monoclea forsteri (Cowan et al. 1992). In this work photosynthetic rate is
measured both in air and in helox (21% oxygen, 79% helium). Since CO 2
diffuses 2.3 times faster in helox, an increased A is obtained at any chosen
CO2 concentration, in he lox compared to in air if a gas-phase diffusion
resistance is present and CO2 is limiting photosynthesis. In M. forsteri, A is
identical in both air and helox indicating the absence of any significant gasphase resistance. Some form of cuticle (resistance re in Fig. 16.2) is now
known to exist, even on leafy mosses, and any increase in cuticular resistance
to water loss will also affect CO2 uptake (Proctor 1981). Most of the
resistance to CO2 diffusion is between the exchange surface and the photosynthetic centers (Nobel 1977). The presence of these large, non-gas-phase
resistances is suggested by the response of A in bryophytes to CO2 concentration. Typically A does not saturate until well above 1000 III CO2 1-1 air
(see Sect. 16.2.6). One value cited for the CO2 diffusion resistance is
95 scm- 1 (Mnium ciliare, Nobel 1977).
The boundary layer resistance can be increased in leafy liverworts and
mosses by arrangement of shoots into clumps or turfs, or by terminal,
nonphotosynthetic structures such as hair points (Proctor 1981). Moreover,
the photosynthetic structures, "leaves", can be arranged in a form of microcanopy that can achieve the following high leaf area indices: 1, Monoclea
forsteri, thalloid liverwort (Green and Snelgar 1982a); 6, Tortula intermedia;
18, Mnium hornum; 20-25, Pseudoscleropodium purum (Proctor 1979);
15, Drummordia prorepens (Vitt 1990); 44, Tortula ruralis; 129, Ceratodon purpureus (Simon 1987). Entire microcanopies would be expected to
achieve higher photosynthetic rates than individual stems through better
light utilization.
b) Ventilated structures, photosynthetic tissue not at the surface (Fig.
16.2).
1. behind a pore: thalloid liverworts (Marchantiales),
2. behind a slit: members of the Polytrichales (parallel photosynthetic lamellae on the upper leaf surface have enlarged, wax-covered terminal cells
that confine gas-exchange to a fine slit between the lamellae).
This grouping contains the endohydric mosses and liverworts, plants that
have variously water-proofed surfaces, often particularly well developed
near gas-exchange pores (SchOnherr and Ziegler 1976; Clayton-Greene
et al. 1985), and a significant internal water-transport pathway. Although
poikilohydric, they are, in many ways, equivalent to homoiohydric higher
plants with "leaves" having an increased internal area to surface area ratio,
for instance 9 for Marchantia foliacea (Green and Snelgar 1982a). According
to Nobel (1977), a higher internal area to surface area should allow higher A
and certainly some of the higher values for A in mosses are those of
Polytrichum species.