The morphological stages of seed development in rimu were analysed by studying microtome sections and by dissections of ovules under a stereomicroscope. Development takes 18 months from the time ovules become visible and losses occur throughout this time. Many empty seed are produced and this has been shown to be due to failure of fertilisation which is usually the result of failure of pollination 12 months earlier. Of the 1,500 ovules examined 21% contained no pollen grains, 47% 1 grain, 23% 2 grains and 9% between 3 and 5 grains. High levels of pollination (mean 1.5 pollen grains per ovule) were found in Whirinaki and Ashley Forests and low levels (mean less than one grain per ovule) in Tautuku and Puketi. Pollination failure was probably caused by wet weather. The percentage of ovules surviving after fertilisation varied between localities, high percentages being found in samples from Saltwater and Ashley Forests and low percentages in samples from Otaki Forks and Pakawau. All ripe red receptacles carried sound seed. Rimu seedlings from 5 provenances (Puketi, Waitakere, Pureora, Westland and Catlins) were used in a variety of growth experiments to examine their responses to different growing irradiances, temperature regimes, photoperiods and seasons. Growth was assessed by growth analysis and direct measurement. Gas exchange studies were made using an infra-red gas analyser and an assimilation chamber in an open circuit system. Photosynthesis and dark respiration rates were measured in rimu and compared with Pinus radiata D.Don. Light saturated photosynthesis rate (g CO₂ absorbed per m² total foliage surface area per hour) of P. radiata (0.79g m⁻² h⁻¹) was more than four times that of rimu (0.18g m⁻² h⁻¹), but the dark respiration rates were not significantly different (P. radiata 0.12g m⁻² h⁻¹, rimu 0.07g m⁻² h⁻¹) (p = 0.05). Light compensation points of the two species were similar, rimu’s being only slightly lower than that of P. radiata (P. radiata 38.8μE m⁻² s⁻¹; rimu 30.8μE m⁻² s⁻¹) and the quantum requirements (number of Einsteins of energy required to fix each mole of CO₂) were also not significantly different (P. radiata 45.5 E mol⁻¹; rimu 53.3 E mol⁻¹). Stomatal resistance to CO₂ of the two species did not vary greatly (P. radiata 8.05 s cm⁻¹; rimu 7.52 s cm⁻¹), but the internal leaf resistance of rimu was more than 13 times as great as that of pine (P. radiata 7.35 s cm⁻¹; rimu 100.0 s cm⁻¹). Growth rate of rimu seedlings grown under varying irradiances showed that 2 year old seedlings could survive at 1% full sunlight, but their growth was negligible (-0.03g g⁻¹ day⁻¹ x 10⁻²) whereas there was little difference between the growth rates of seedlings grown at 17% and 42% sunlight (0.26 and 0.24g g⁻¹ day⁻¹ x 10⁻² respectively). On the other hand small first year seedlings had significantly greater mean relative growth rates in the later part of the growing season (February-March) when grown in higher irradiances. Growth rates were 0.93, 1.21 and 1.62g g⁻¹ day⁻¹ x 10⁻² for plants grown at 28%, 60% and 73% sunlight respectively. Chlorophyll concentrations of seedlings grown at low irradiance (140μE m⁻² s⁻¹) were significantly higher (0.69 mg g⁻¹ fresh weight) than concentrations in plants grown at 385 and 650μE m⁻² s⁻¹ (0.28 and 0.26 mg g⁻¹ fresh weight respectively). However, medium irradiance plants had a somewhat higher relative growth rate (0.83g g⁻¹ day⁻¹ x 10⁻²) while low and high irradiance plants grew at similar rates to each other (0.59 and 0.60g g⁻¹ day⁻¹ x 10⁻² respectively). Light saturated photosynthesis rate of medium irradiance plants was lowest (0.38g m⁻² h⁻¹) while those of low and high irradiance plants were the same (0.43g m⁻² h⁻¹). Net photosynthesis rates of plants from the three growing irradiances all responded to increasing temperature in a similar way and the optimal temperature range for photosynthesis was between 18° and 22°C. With increasing temperature there was a trend towards increased relative growth rate in eight month old Pureora provenance seedlings: 1.52, 1.70 and 1.73g g⁻¹ day⁻¹ x 10⁻² for plants grown in 17°/9°C, 22°/14°C and 27°/19°C regimes respectively. The medium regime, however, gave best growth in fifteen month old Puketi provenance seedlings; 1.04 compared with 0.93 and 0.82g g⁻¹ day⁻¹ x 10⁻² in the warm and cool regimes respectively. These differences were believed to be due to ontogentic stage rather than to provenance. Seedlings of both provenances became brown in the cool regime. 5°C nights with 18°C days also caused two year old seedlings to become brown. Chlorophyll a + b concentrations decreased with decreasing night temperature (0.50, 0.38, 0.32 mg g⁻¹ fresh weight at 20°, 12° and 5°C nights respectively). Relative growth rates increased with increasing night temperature (0.78, 0.98 and 1.08g g⁻¹ day⁻¹ x 10⁻² at 5°, 12° and 20° nights respectively) as did light saturated photosynthesis rates (0.16, 0.18 and 0.19 gm⁻² h⁻¹). Internal leaf resistance to CO₂ increased greatly with decreasing night temperature (58.8, 100.00 and 142.9s cm⁻¹ for plants grown at 20°, 12° and 5°C nights respectively) and the Q₁₀ of dark respiration rate increased slightly with decreasing night temperature (1.7, 1.9 and 2.0 for plants grown at 20°, 12° and 5°C nights respectively). Several Puketi, but no Waitakere provenance seedlings, died after several months in 5°C nights. Different provenances of rimu seedlings responded to photoperiod in different ways. In Puketi plants 20 hour and 15 hour days caused significantly more total shoot growth than 10 hour days. This trend: long day > medium day > short day, was repeated in the Pureora provenance, but there was no obvious trend in shoot growth in the Waitakere or Westland provenances. Shoot surface areas of all four provenances, however, followed the trend: long day > medium day > short day, with significant differences in both the Puketi and Pureora provenances, and relative growth rate, measured only in the Waitakere provenance, followed the same trend: 1.51, 1.43 and 1.30 g g⁻¹ day⁻¹ x 10⁻² for plants grown in 20 hour, 15 hour and 10 hour days respectively. Winter dormancy in rimu seedlings is imposed by low mean temperatures and can be broken by increased temperatures without a period of chilling. Provenances from four localities grown in a nursery in Rotorua all ceased growth at the same time in winter (June) and their seasonal growth patterns were similar. Provenances from warmer climates (Puketi, Waitakere and Westland) grew as well in Rotorua as the Pureora Provenance from a colder upland climate. All seedlings became brown before growth ceased in winter and remained brown until November or December. There were some distinct morphological differences between certain of the provenances. Results of the various growth experiments are discussed in relation to the ecology of the species.