This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Seeds of Batrachian Ranunculi, which come under the old aggregate Ranunculus aquatilis, have been found in some deposits, such as Pre-glacial, Early Glacial, Interglacial, Late Glacial, and Neolithic beds. To-day the distribution of this aggregate extends over Europe, West Asia, the Himalayas, N. America, or the Warm Temperate Zone. In occurrence it is absent from Cornwall in the Peninsula, South Hants and E. Sussex in the Channel province, but occurs throughout the Thames province; in Anglia it is absent from Northampton, but present in the whole of the Severn province, and absent from Notts in the Trent province, occurring in Carnarvon and Denbigh only in Wales, in S.W. and N.E. Yorks, Tyne province, and in Scotland in Dumfries, Kirkcudbright, Haddington, Edinburgh, Fife, Stirling, Perth, Forfar, Clyde Islands, Sutherland, and the Hebrides. It is also found in Ireland.
In all the Water Buttercups the leaves are much divided. There are two types. Those that float are rounded with 3-6 wedge-shaped lobes, which are inversely egg-shaped or rounded. The submerged leaves, on the other hand, are linear, much divided, with numerous fine segments. There are other modifications of this type, some, as R. fluitans, having no floating leaves, but very long hair-like leaves (submerged) only when growing in quickly-flowing water. At the other extreme are plants, as R. hederaceus, with few if any submerged or thread-like leaves, and only the rounded floating types of leaf. The types with two kinds of leaves may become stranded on land and then adapt themselves to such conditions, though as a rule the submerged-leaf type cannot succeed on land or the floating leaf under water.
This species is an aquatic like other Batrachian Ranunculi, with thread-like submerged leaves, hydrophilous, preferring the still water of a pond or lake to that of running water, thus differing from Ranunculus penicillatus. It forms clusters and groups growing in the centre of the pond, and is associated with Water Cress, Water Persicaria, Pondweeds, Duckweed, Water Plantain, and the semi-submerged Celery-leaved Water Crowfoot.
Water Fennel is adapted, like all aquatic plants, to growth in water, with linear leaves and slender stems which float readily in the water.
Water Fennel is distinguished from other Water Buttercups by its small flowers, its rigid leaves which do not collapse when removed from the water, and the short compact flower-stalk.
When floating, the flowers do not rise at the utmost more than 2 in. from the surface. It is in flower from April to August, and is perennial.
In the aggregate R. aquatilis the yellow base of each petal acts as a honey-guide, and on it stands an oblique tubercle or wart-like projection, with a honey-secreting depression, which serves as a gland and receptacle for nectar. There are few anthers (8-20 stamens), which open, an Allied Species to show the Habit in succession when the flower opens, being turned to the centre.1 The anther-stalks later, twisting over the honey-glands, become immersed in pollen, the stigmas developing, and coming in contact with the pollen on the anthers. Visitors alight on the middle of the flower or on its edge, and cause self-pollination or cross-pollination. When the surface of the water rises the flowers remain submerged, and are self-pollinated. The visitors are Diptera, Syrphidae, Eristalis, Helophilus, Chrysogaster, Muscidae, Scatophaga Hymenoptera, Apidae, Apis, Bombus, Coleoptera, Chrysomelidae, Helodes. The flower is scented.
Photo. G. B. Dixon - Water Buttercup (Ranunculus Fluitans, Lam.)
1 Next day the outer stamens move outwards and another whorl takes their place, and so on till all have opened.
The plant is dispersed by the agency of water or by animals. The achenes when ripe either fall to the bottom or float about on the surface of the water. The stems, etc., are likely to be dispersed by wading-and swimming-birds.
Water Fennel is a Hydrophyte and aquatic belonging to the submerged and floating-leaf associations.
No fungi infest the plant, nor do insects feed upon it.
Pliny invented the name Ranunculus, which is a diminutive of the Latin rana, a frog, and so a little frog, the Ranunculus affording a habitat for little frogs in early spring; while trichophyllus is from Greek thrix, hair, phyllus, leaf. In Buttercup or Buttercop, cop means a head. Water Fennel, in allusion to its leaves resembling Fennel, is the only vernacular name.
Unlike the terrestrial buttercups, which cause blistering, this plant is innocuous. Cattle have been fed on these Water Crowfoots by the Avon banks, and when freshly taken from the river cows enjoy it. In reference to the amphibious forms as a whole, Dr. Pulteney (a Leicestershire botanist) showed that they are highly nutritious.
Essential Specific Characters:5. Ranunculus trichophyllus, Chaix. - Submerged leaves not collapsing, petals 7-veined, buds globose, flowers white, with a yellow centre, carpels compressed.