Description

The D.rotundifolia is a perennial forb, 6-8 cm wide and 1-1.5 cm high (without the flower). It has from 3 to 8 leaves, arranged in a basal rosette, that generally are horizontally extended, even though sometimes it is possible to observe upright leaves.

Two similar species are D.anglica and D.intermedia; you can distinguish it from D.rotundifolia because they have more elongated leaves. In some sites it is possible to observe D. x obovata, a natural hybrid between  D. anglica and D. rotundifolia.

3.1 Leaves

The petiole is quite long (from 2 to 5cm) and it is covered by very minute white hairs; the leaf has a roughly rounded shape (0.5 - 1cm diameter); often it is a little broader than long. The upper surface of the leaves has red "hairs" or "tentacles"; each hair has a red gland at its end, covered by a transparent, sticky secretion that is used to trap the preys.
The number of tentacles vary from ~100 to ~200; the ones on the center of the leaf are short and upright and they have green pedicels, instead the tentacles on the margin are longer and inclined outwards (i.e. horizontal), their pedicels are usually reddish. Few tentacles spring between the base of the leaf and the petiole; they are the longest ones.

3.1.2 Tentacles

3.2 Roots

The roots are shallow, usually near 3-4 centimeters. They consist of a taproot with few absorbent hairs; they are replaced every year. Their main function is to provide to the plant enough water to maintain the sticky liquid on its tentacles (remember that D.rotundifolia is often fully exposed to sun); obviously they are also able to absorb nutritious matter from the soil. It has been proved that the plant can survive even merely with the nutrient of the soil, but it grows slower and smaller.

3.3 Flowers

The inflorescence structure is a one-sided raceme, simple or forked. There are from 2 to 15 small flowers an a 10-25cm long stalk. The very long stalk is a characteristic found in many carnivorous plants; it is required to keep away the pollinator insects from the leaves (carnivorous flowers don't exist; some flowers, as the ones of Arum maculatum, "trap" the insects but only for shot periods of time, for reproductive purposes).
The flowers generally are white (or sometimes light pink); they have five sepals (4mm long), five oval petals (6-8mm) and five stamens (5-7mm); the pistil has three styles. They require abundant exposure to sun to open.

3.4 Fruit

The fruit is an oval capsule that contains numerous small seeds. The light brows seeds are finely striate and shiny; they have a fusiform shape and they are 1,5-1,8mm long and 0.2mm wide. The seeds have an inflated area at their end so they are able to float on water for some days.

Biology

4.1 Seasonal developement

The plant reproduce vegetatively of by seeds. It flowers in june, july, august or september; the month depends from the place (in Italy it flowers in august).
The inflorescence last for some weeks; during the day the flower are cross pollinated by insects and at evening, when they close, they self-pollinate. The seed are dispersed in the following weeks.
During the fall the plant form a hibernaculum (i.e. a tight group of leaf primordia), the leaves and the roots die. The next spring, during april or may, the hibernaculum opens and the plant begin a new life cycle.

4.2 Predation

D.rotundifolia compensate the nutrients-poor soil with the ability of catching and absorbing insects. When a prey touch the tentacles it is trapped by the sticky liquid secrete by the glands; it movements put in action also the surrounding tentacles that slowly bend on it. In some cases also the leaf becomes very incurved, assuming a cup-like shape. The tentacles needs from one to five hours to curve and when they are inflected the glands increases the quantity and the quality of the secretion (it becomes more abundant and more acid and it is supposed to have also an antiseptic power ). After a certain number of days, from one to seven, the organic matter of the prey is absorbed and the tentacles begin to re-expand. The secretion of the glands may diminish or stop during the re-expansion, but when the process is complete they begin to re-secrete and the leaf is ready to catch another insect.

4.3 Effects of lighting

It is interesting to see the effects of the lighting on this plant. It needs abundant sunlight; the plants exposed to the sun are usually bigger than the shaded ones and they have a different color (they are red, instead the shaded plants are more greenish). The invasion of bigger plants, like woody vegetation, can led to the disappearance of D.Rotundifolia, since it is very small and then easily shaded.

4.4 Interactions with habitat and other species

The sundew is able to colonize "difficult" sites like bogs disturbed by peat mining of by fire. Nonetheless, the altering of the site conditions might led to its disappearance. Clearing and drainage of peat bogs, agricultural purposes, fertilization and fire suppression influence negatively of the diffusion of the plant.
The D.rotundifolia is a food source for animals, e.g. moose, that eat the plant. Other animal like the bog-dwelling ants eat the preys caught by the plant.

Habitat

D.rotundifolia lives in temperate areas; it needs a definite winter season to complete its life cycle (during the winter, it rests and it forms the hibernacula). It tolerates temperatures as low as -10 Celsius degrees.

It is found mainly into bogs; other habitat are swamps and areas near lakes or ponds. It is often associated with sphagnum mosses.
The soil of these areas is poor of nutrients (calcium, nitrogen, phosphorous), it is usually acid (Ph<7) and it is very wet (the water table is from 2 to 40 cm below the soil surface). The plant can tolerate flooding even for several weeks, instead it tolerates dry periods only for few days.

Distribution

America:
Canada, U.S.A ( Alabama, Alaska, California, Colorado, Connecticut, Delaware ,Georgia, Idaho, Illinois, Indiana, Iowa, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Montana, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Oregon, Pennsylvania, Rhode Island, South Carolina, Tennessee, Vermont, Virginia, Washington, West Virginia, Wisconsin )

Europe:
France, Germany, Great Britain, Ireland, Italy, Netherlands, Norway and other countries

Asia:
Russia (maybe also in China and India)

Other:
South Africa, South America

6.1 Detailed Distribution Maps

U.S.A. distribution map

Darwin's studies

Charles Darwin did extensive studies about the carnivorous plant, in particular about Drosera rotundifolia, and he wrote the book "Insectivorous Plants".

These are some quotes from the book. You can download the entire book here.

"During the summer of 1860, I was surprised by finding how large a number of insects were caught by the leaves of the common sun-dew (Drosera rotundifolia) on a heath in Sussex. I had heard that insects were thus caught, but knew nothing further on the subject. I gathered by chance a dozen plants, bearing fifty-six fully expanded leaves, and on thirty-one of these dead insects or remnants of them adhered; and, no doubt, many more would have been caught afterwards by these same leaves, and still more by those as yet not expanded."

"Many plants cause the death of insects, for instance the sticky buds of the horse-chestnut (Aesculus hippocastanum), without thereby receiving, as far as we can perceive, any advantage; but it was soon evident that Drosera was excellently adapted for the special purpose of catching insects, so that the subject seemed well worthy of investigation.
The results have proved highly remarkable; the more important ones being--firstly, the extraordinary sensitiveness of the glands to slight pressure and to minute doses of certain nitrogenous fluids, as shown by the movements of the so-called hairs or tentacles; secondly, the power possessed by the leaves of rendering soluble or digesting nitrogenous substances, and of afterwards absorbing them; thirdly, the changes which take place within the cells of the tentacles, when the glands are excited in various ways."

"It is necessary, in the first place, to describe briefly the plant. It bears from two or three to five or six leaves, generally extended more or less horizontally, but sometimes standing vertically upwards. The leaves are commonly a little broader than long. The whole upper surface is covered with gland-bearing filaments, or tentacles, as I shall call them, from their manner of acting. The glands were counted on thirty-one leaves, but many of these were of unusually large size, and the average number was 192; the greatest number being 260, and the least 130. The glands are each surrounded by large drops of extremely
viscid secretion, which, glittering in the sun, have given rise to the plant's poetical name of the sun-dew."

History and uses