In Lake Opinicon, Frontenac County, Ontario, the temperature gradient at midday from 10 cm below the water surface to soil, in clear weather in August, was found to be 10 Celsius degrees (C°)/m in shallow water where there was a large macrophyte biomass and only 0.2 C°/m in deeper water with fewer plants. There was a direct correlation between the log of the biomass per cubic metre and the steepness of the gradient. Reference is made to temperature gradients developed in calm weather during phytoplankton blooms in Scottish lakes.The biomass of aquatic macrophytes growing in a heated greenhouse in tubs with a capacity of 1 m3 increased from early January to the end of February. With increased biomass, the temperature gradient became larger. The size of the gradient correlated with the incoming radiation. Temperature gradients within large populations of the pondweed Potamogeton richardsonii were enhanced by low temperatures near the bottom.Diurnally produced temperature gradients with an algae bloom in the same tubs in the open in late June showed strong correlation with solar radiation, 8 C°/m with cumulative radiation of 22 MJ/m2 and 4.6 C°/m with 12 MJ/m2. The gradients produced by a population of P. richardsonii with tips 17 cm from surface were from 5.2 to 3.4 C°/m but showed less correlation with radiation when the wind velocity was above a threshold value. No gradient was produced in tubs of water without plants.

On clear days the temperature in a mat of floating Lemnaceae was 40 to 11°above that of the surface of the open water. The temperature of the environment 2 cm above or below the plant surface closely followed the surface temperature's quick response to changes in net radiation. The plant surface reflects more energy and transmits less than the open water, therefore there is less fluctuation in temperature in the water o cm and more below the floating plant surface. This results in a diurnal thermal stratification. At the surface of the open water changes in temperature tended to be smaller and slower, related to the air temperature; and, with wind disturbance, the water profile maintained relatively homogeneous temperatures.

Instratifiedlakeswithhighlightpenetrationthemaximumdepthatwhichmacrophytesoccurisfrequent lylimitedbytemperature Atthisdepthavarietyofspeciesmaybefound Ontheotherhandwhenthe clarityofwaterlimitsthelightpenetrationandthetemperatureatdepthissufficientforgoodplantgrowth theplantsoccurringatgreatestdepthareonesthatdonotrequirephotosyntheticoxygenforrootgrowth Suchplantsincludethe CharalesIsoetesUtricularia and Ceratophyllum

Although the prirnordia of all types of leaves in IIippliris show no distinguishing characteristics until they are 50 microns long, their further development is strongly influenced by their environ~nent. A t maturity they can, for convenience, be thought of as belonging to one of five types, or some intermediate of these Types. The five types of leaves are: rhizome, juve~lile aquatic, adult aquatic, juvenile aerial, and adult aerial. Field and experimental studies indicate that the leaf form in Hippziris is influenced by light and the water relations between the plant and its environment. There is great uniformity of foliar morphology on plants from stocl;s of diverse origin when grown under ~~n i f o r m condit~ons, and there is wide diversity of foliar n~orphology on plants of the same stock material when grown under different environments in the laboratory. TIILIS the taxono~nic subdivision of genus Hippziris into species, varieties, and forms using leaf morphology as the main criterion appears questionable. IIbIanuscript

In the crowded parts of patches of Hieracium floribundum Wimm. and Grab., which were established for at least a decade in an ungrazed pasture, 7–10% of the population (3700 individuals/m2) flowered. Of the plants that flowered, 94% were alive a year later, but only 5% of these flowered. Abortion of flower heads was common; one half of all those which were initiated in early June had aborted by flowering time in early July. Freshly dispersed seed had a viability of 57%, which was reduced to 17% a year later, after its storage close to the soil surface. Less than 6% of the viable seed was innately dormant. Dry, laboratory-stored seed retained its viability for the year but was slower to germinate than soil-stored seed. In field conditions, germination was temperature inhibited during most of the growing season. Maximum daytime microsite temperatures of less than 32 °C, which is necessary for germination, were found to occur only in early spring and late fall. Successful seedling establishment accounts for 1% of the individuals in a crowded population. Based on the maximum sexual reproduction from plants in crowded populations, a seed has a probability of 1 in 20 000 of becoming an established seedling. A model of population dynamics in a high-density patch traces the fate of seedling establishment, surviving adults, and stolon-derived rosettes for a single year.