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Physiological integration of Thalassia testudinum short shoots enables clones to function at a higher level of physiological organization than that of the short shoots themselves. Shaded short shoots connected to non-shaded short shoots had blade growth rates and proximate organic constituent levels equal to non-shaded controls. Shaded short shoots physically isolated from neighboring short shoots had blade growth rates and organic constituent levels different from both controls and shaded short shoots connected to non-shaded short shoots. Support of shaded short shoots appeared to be primarily from older short shoots connected to the shaded short shoots. The amelioration of localized light limitation results in an increased ability of clones to persist in heterogeneous environments. This increases the probability of the clone later encountering more favourable sites through a wider physical spread.

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Photosynthesis versus irradiance ( P vs 1) parameters in the shoalgrass Halodule wrlghtii Ascherson were calculated from measurements of oxygen evolution collected in situ, using entire plants in Laguna Madre, Texas, USA. Eleven expenmental incubations were performed from May 1989 to April 1991 using pulsed oxygen electrodes that collected data continuously under natural daylight conditions For comparison with in situ measurements of photosynthesis, P v s 1 parameters were calculated from laboratory measurements on blade segments incubated In a small volume chamber. For field plants, average saturation 111-adiance (I,) was 319 pm01 m -2 S -' , photosynthetic capacity (P,,,) was 374 pm01 O2 g -l dry wt (dw) h -' , and relative quantum efficiency (a) generally ranged from 0.5 to 1.6 pm01 O2 g-I dw h -l (pm01 photons m -2 S-').'. Whole plant respration was 70 pm01 g ' dw (leaf) h ' , and compensation irradiance (I,,) was ca 85 pm01 m -2 S -' Chlorophyll a concentrations averaged 12.8 mg g -' dw. and the mean chl d:chl b ratio over the 2 yr period was 2 2 For blade segments incubated in the laboratory under similar temperatures to field plants and at corresponding chlorophyll concentrations. P,,, was not significantly different but a was signif~cantly higher, ranging from 3.4 to 5.3 pm01 O2 g -' dw h -' (~.lmol photons n r 2 S-').' The higher a values resulted In significantly lower estimates for 1, (mean of 101 pm01 m -2 S-'). The higher U in laboratory plants was largely related to the perpendicular orientation of leaf tissue to a directed hght field, which is not reflective of natural conditions for H. wrightii. The significant difference in 1, values calculated from incubations performed in the laboratory and field has profound effects on model calculations of H,,,, the duration of irradiancesaturated photosynthesis, and thus predictions of the minimum llght requirements required to sustain growth in H. wrightii. Consequently, the application of laboratory-derived 1, values for H. wrightii would result in overestimates of its maximum depth limlts and rates of area1 primary production