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Hydrocharis morsus-ranae L. is a Eurasian aquatic plant introduced to North America. It spreads primarily by the rapid production and growth of stolons and overwintering turions but still flowers extensively. The white, trimerous flowers of H. morsus-ranae are emergent and unisexual, with male flowers clustered in a cyme of up to five buds and females always solitary. Both sexes of flowers, once open, last a single day, with individual male flowers opening sequentially from a given inflorescence but not necessarily on successive days. First openings in the corollas of flowers were visible by 0730 and maximum expansion was usually reached by late morning. Anthers dehisce and stigmas become receptive by 0930 with both reaching maxima by midday. The flowers produce a sweet nectar and scent which attract, and are easily accessible to, a wide variety of insects visiting the open bowl-shaped flowers. The most abundant insect visitors to the flowers were Homoptcra (Aphididae) and Hydrellia and Notiphila spp. (Diptera: Ephydridae). Both groups were found to carry small pollen loads and because of their erratic anthophilous behaviour were not considered important in pollination. Although fewer in number, the more specialized hover flies, Toxomerus marginatus (Say) (Diptera: Syrphidae), and solitary bees, Dialictus sp. (Hymcnoptera: Halictidae), were considered more likely to be the primary pollinators. After pollination and the day after anthesis, female flowers are drawn underwater by pedical recurvation and 4–6 weeks later mature into globose berrylike fruits. Fruit-sets were 38.3% in naturally pollinated flowers, 96.5% in bagging experiments with hand-pollinations, and absent in bagging experiments for agamospermy with unopened flowers. These results substantiated that poor fruit-set in some populations was caused by a lack of effective pollinators. Seed densities of 250/m2 were still estimated with the above data.

This study of seed and seedling morphology in the aquatic monocotyledon Hydrocharis morsus-ranae L. (Hydrocharitaceae) represents part of a continuing study of sexual reproduction in this species. Scanning electron microscope studies of the seeds showed them to have testas covered with hollow spiraliform tubercles. Germination occurs when the radicle and cotyledon of the embryo from the exalbuminous seed elongate, splitting the tuberculate testa. The buoyant embryo then rises to the surface with the first foliage leaves emerging from a highly modified cotyledonary sheath. This makes early growth in the seedling look strictly hypocotyledonary. Both the radicle and cotyledon discontinue growth by the two-leaf stage in the seedlings. The young seedlings undergo several growth phases. At first they look Lemna-like in habit and then, subsequently, very similar to germinating turions of the same species. Several methods are discussed to help distinguish between germinating turions and seedlings. During a subsequent field study of a H. morsus-ranae population, only two germinating seedlings were discovered. Several reasons for the absence of seedlings in this population are discussed.

Roots of water hyacinth (Eichhornia crassipes (Mart.) Soims.) and frogbit (Hydrocharis morsus-ranae L.) when grown In solution cultures adjusted to pH 3.5, 4.0, 5.0, 6.0, and 6.5 did not suffer decidedly adverse, direct effects from hydrogen ions except at extreme acidities represented by a pH value of less than 4. During cool weather the plants were able to grow at a somewhat higher concentration of hydrogen ions than was possible in the warm summer months. The boundary between good growth and toxicity was a narrow one. Extreme acidity, short of the critical point, inhibited the growth of the roots by decreasing cell division and cell elongation. In the roots investigated, cell division at pH 3.5 was reduced to one-half of the rate at pH 5.0 and this reduction accounted for three-quarters of the inhibition in the growth of the roots. When grown in a highly acid medium, root cells acquired a resistance to the hydrogen ions. Water hyacinth meristem cells from the highly acid media exhibited a pronounced resistance to hydrochloric acid. It is suggested that this increased resistance is due to the cells having acquired a greater impermeability to the acids. Determination of the pH of expressed root juices revealed that, although all of the saps were decidedly less acid than were the media in which the plants were grown, roots grown in an extremely acid medium had a slightly more acid sap than those grown in a less acid one. A comparison of rates of streaming and the action of protoplasts during plasmolysis indicated that extreme acidity in the growth medium increased the viscosity of the cytoplasm of root cells. No significant difference was noted in the osmotic pressure of the root cells from the different cultures. The absolute permeability of frogbit root hairs to thiourea, as calculated from the formula [Formula: see text], did not vary significantly from.0012 millimoles per square centimeter of surface per hour per molar concentration difference for plants grown within the range of pH 3.6 to pH 6.0.

Extension growth of the chlorophyll-containing roots of Hydrocharis morsusranae was inhibited by 0.5 p.p.m. of 3-(4-chlorophenyl)-1,1-dimethylurea (monuron) whereas concentrations close to the water saturation point of 230 p.p.m. were required to inhibit extension growth of the non-chlorophyll-containing attached roots of Zea mays and Phleum pratense and the detached roots of Pisum sativum.A total of 15–20 μg of monuron per gram fresh leaf applied through the cut petiole of detached primary leaves of Phaseolus vulgaris inhibited the increase of dry matter by 90% and suppressed transpiration 40–50%. Internal concentrations of 1–2 μg/g of monuron produced simultaneous enhancement of dry matter increase and of transpiration but concentrations of 5–10 μg/g produced a suppression of dry matter increase concurrently with an enhancement of transpiration. Age of leaf and the time of year in which the plants were grown altered the critical internal concentration levels required to affect dry matter increase and transpiration.Analysis of detached leaves treated with 15–20 μg/g monuron indicated a marked suppression of the formation of non-water-soluble carbohydrate, a slight suppression of the formation of water-soluble nitrogen, but little or no effect on water-soluble carbohydrate or on non-water-soluble nitrogen.In detached leaves o-phenanthroline, 3-phenyl-1,1-dimethylurea, and 3-(3,4-dichlorophenyl)-1,1-dimethylurea resembled monuron closely in symptom development and in their effect on dry matter production and transpiration. Iodoacetamide, 2,4-dinitrophenol, and 8-hydroxyquinoline each produced some effects similar to monuron but differed from it in certain respects; Thiourea, sodium diethyldithiocarbamate, sodium fluoracetate, ethyl-NN-diphenylcarbamate, and hydroxylamine hydrochloride were without noticeable effect.