Teasel (Dipsacus) - a semi-carnivorous plant
Above: Wild or Common Teasel, Dipsacus fullonum, Teasel family (Dipsacaceae).
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The teasel (Teazel, Dipsacus) is a striking grassland dicotyledonous plant and flower. Native to Europe, Asia and North Africa, Wild Teasel is invasive in North America (Rector et al. 2006) and may be found in savannas and fields, but prefers wet places such as wetlands, lake borders etc. It is tolerant to high salinity and so also occurs along roadsides.
The club-shaped plant,
up to about 1 to 2.5 m tall, is immediately recognizable by its large
spiny and club-like flower head, which 4-10 cm long. Recall that an inflorescence (main flowering shoot) may
bear one or more flowers, many in this case, and that each flower is
born on its own stalk, axis or shoot called a pedicel (a very short stalk ends in
the bracts subtending each flower so we can also think of this
'pedicel' as a peduncle or secondary peduncle if we count the main
flower-head bearing axis as a peduncle). In the photo above you can
see the inflorescence - its is the single terminal shoot bearing the
flowers that emerges above the final whorl of green leaves (which we
can call bracts). This is a monopodial
(bearing a single main axis) and so is called a raceme (a cyme
branched inflorescence). The inflorescence is also terminal, growing
at the end of a vegetative shoot. This single axis bears side-branches
which are the individual flower stalks (pedicels) which are so
compressed and shortened in this case as to be unnoticeable (we can
say that the flowers are stalkless if there is no shoot length between
the inflorescence and the first whorl of flower parts) but the flower
stalk continues as the main axis bearing the various parts of the
flower. Such a compacted inflorescence is also called a flowerhead
(seedhead) or anthode or capitulum. Individual flowers
in the flowerhead can be referred to as florets. The stalk to
the whole flowerhead is the inflorescence stalk or peduncle.
Teasel is often described as a biennial, but when conditions are sub-optimal it may require additional years to reach maturity and so is more accurately described as a short-lived monocarpic perennial. A monocarpic plant flowers once then dies. The main axis bears the largest and oldest anthode which flowers by about midsummer followed by the secondary anthodes. The flowers are pollinated by a range of insects, including bumblebees and macrolepidoptera (larger butterflies and moths). The seeds mature in autumn and most fall in the autumn, but some do not fall until the following spring.
The vast majority of seeds (99.9%) fall within 1.5 m of the parent Teasel. Longer distance dispersal then occurs by water (floodwater/runoff, ditches and streams). The 'seeds' are actually the fruit of the individual flowers. Each fruit is a single-seeded achene and is enclosed by the persistent epicalyx. The epicalyx has 4 angles in cross-section and an additional ridge of folded tissue on each of the 4 faces and is covered in minute hairs. The epicalyx makes the fruit less dense and so improves its ability to float in water. The fruit may float for up to 22 days without losing viability. The single seed inside contains nutritive endosperm enclosing the embryo.There are up to 40 seedheads per plant with as many as 2000 seeds per head.
The surface of the anthode (the clinanth or receptacle) is covered in spinous scales (receptacle scales, one per floret). Beneath the flowerhead is an involucre (a saucer-like cluster or rosette of bracts, the pericline) of spiny bracts / phyllaries.
A characteristic of the Dipsacaceae is the epicalyx, which arises at the base of the ovary and encloses the ovary in a tubular structure (involucel). The involucel has 8 furrows and 8 ridges, is covered with tiny hairs and persists to enclose the fruit. There are 4 stamens and the calyx (sepals) arise from the top of the ovary, forming a crowning cup or funnel with 4 teeth. The corolla is a 4-lobed petal tube.
The flowers are protandrous: the male organs ripen first, followed by the female parts. This helps prevent self-pollination. The two upper stamens mature before the lower two. There are no asexual or vegetative modes of reproduction.
Epicalyx and calyx: the calyx is a whorl of sepals which surrounds or occurs beneath the whorl of petals in a flower and in some flowers another whorl of leaf-like structures occurs just beneath or outside the calyx - this is the epicalyx. It consists either of bracteoles or sepal stipules.
involucel: bracts supporting a flower or flowerhead, by forming
a saucer or cup-shaped shield beneath it, is called an involucre. In
Dipsacaceae, smaller bracts enclose each individual flower (floret) to
form a secondary involucre or involucel.
bracts are additional leaf-like structures borne either on the flower
stalk (which is very short and compacted in this case) in which case
they are typically small and called bracteoles, or at the base of the
flower stalk (bracts). Both types occur in the teasel, and both bear
spines. The bracteoles often, as in the teasel, form a cup or
Stipules: leaves, and sepals (modified leaves) sometimes contain stipules (typically two per leaf in a monocotyledon, and usually absent or 1-2 per leaf in a monocotyledon). They occur at the leaf base, on the leaf stalk or at the base of the leaf stalk. They serve to protect developing structures in the bud, such as a second leaf that is still developing. They may be so enlarged as to appear superficially like another pair of leaves.
the lavendar-colored flowers open first in an equatorial belt around
the middle of the flower-head. Flowering then proceeds to both ends,
forming two belts of open flowers moving towards the ends (see the
first photograph above) as the older flowers turn to seed. Notice
the whorls of long pointed leaves further down the vegetative shoot.
Visit www.microscopy-uk.org for a beautiful article on the Teasel, including close-up views of single flowers.
Below: the dried flower-head persists as the seeds develop, into the Autumn (Fall) until the seeds disperse.
These dry fruit-heads (borne on what is now the infructescence rather than the inflorescence). The spines are quite capable of piercing human skin and these heads were once used extensively for teasing out fibers in the processing of fabrics like wool. They were also used to 'raise the nap' or lift the fibers to make the fabric 'woolly'.
Teasel is Semi-Carnivorous!
Another distinctive feature of the Teasel is the whorls of long vegetative leaves borne at intervals along the main shoot (stem leaves). Each leaf is up to 20 to 40 cm long, bears spines along the underside of its midrib, and is tapered (lanceolate). The leaves are not borne on stalks, but instead the paired leaves of each whorl fuse into a cup-like structure around the stem, which can trap considerable quantities of rainwater, which the plant may later utilize, but which also traps many insects which decompose in the water. These water reservoirs give the name Dipsacus, from the Greek 'to thirst'. Experiments have shown that nutrients from the decomposing insects are absorbed by the plant, increasing the number of seeds that are produced. Thus the Teasel is partially carnivorous (insects are not apparently essential, but are a utilized source of nitrogen).
See also carnivorous plants.
Water reservoirs trapping nutrients.
The seeds germinate in spring and summer. Wild Teasel develops a basal rosette of leaves in its first year, supported by a large taproot which stores food reserves. Only when sufficient food reserves have been amassed will the plant than bolt, putting up a main axis and flowering. After flowering once the plant dies.As a rule of thumb the rosette must reach a minimum diameter of about 30 cm before bolting will occur.
There is some historical confusion in the taxonomy of teasel. Wild Teasel (Dipsacus fullonum) was formerly called Dipsacus sylvestris (= D. fullonum ssp. sylvestris) and the cultivated form, Fuller's Teasel or Dipsacus sativus, was formerly called Dipsacus fullonum (or D. fullonum ssp. sativus). Whereas the Wild Teasel has spiny scales with straight points that exceed the flowers in length, in Fuller's teasel the spines are recurved and about equal in length to the flowers. The involucral bracts in Wild Teasel are prickly ascending leaves exceeding the length of the flowers and often the height of the anthode, whereas in Fuller's Teasel they are not prickly and spreading or horizontal rather than ascending and shorter than the flowers. The specimens shown here are clearly Wild Teasel.
Rector et al. 2006. Prospects for biological control of teasels, Dipsacus spp., a new target in the United states. Biological Control 36: 1-14. Available at: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.519.6912&rep=rep1&type=pdf
Werner, P.A. 1975. The biology of Canadian weeds. 12. Dipsacus
sylvestris Huds. Can. J. Plant Sci. 55: 783-794.
Scabious (Knautia and Scabiosa)
Knautia arvensis (Field scabious) is also a member of the Teasel
family (Dipsacaceae). The
buds and seedheads of scabious makes the relationship of scabious to
Teasel much more obvious.This plant has a perennial rootstock.
The involucre of phyllaries occur in two rows and the leaf-like
phyllaries are ovate (egg-shqaped in contour) tending towards
being lanceolate (shaped like the head of a lance).
Each flower (floret) within the flowerhead (anthode or capitulum) consists of a 4-lobed corolla (petal-tube) which is asymmetric in the outermost flowers which are radiant (the corolla being extended into a ray). An involucel (epicalyx) encloses each ovary, persisting in fruit. The involucel has 4 longitudinal ridges and with a funnel of dense hairs at the apex and is covered in tiny hairs. The calyx, borne on top of the ovary, has 8 long bristles (see the green seedheads in these photographs). These bristles are present in the fruit but are reportedly deciduous (presumably they eventually detach and fall off).
Above: Field Scabious (Knautia arvensis).Petal color: pale lilac / bluish-lilac. This plant reaches one meter in height and occurs on dry banks, field edges and in waste places.
Above: Field Scabious fruiting.Note the 8 long bristles of the calyx which crowns the ovary.
Above: Flower buds of Field Scabious.
Below: Small Scabious (Scabiosa columbaria) is another member of the teasel family. These pictures of Small and Field Scabious were from the same sight (chalk grassland) where both types were seen growing together. Small Scabious has corollas with 5 lobes, whereas the corolla of Field Scabious is 4-lobed. Field Scabious also has more coarse hairs, whilst in Small Scabious the size difference between the outer florets and the inner florets is more obvious.
Small Scabiousis a smaller, less stout plant reaching 60 cm (2 feet) in height. The stem leaves are generally deeply pinnatifid (divided by clefts that extend more than half-way to the midrib in a feather-like manner) but are very variable (the lobes may also be divided into secondary lobes to give a bipinnate leaf). Basal leaves may be undivided. The corolla has 5 lobes, is blue-lilac (occasionally lilac) in color, and the outer flowers are radiant. The involucre consists of a single row of strap-shaped phyllaries. The involucel (epicalyx) has 8 ridges covered in tiny hairs and is expanded at its apex into a membranous funnel. The calyx on top of the ovary consists of a tubular stalk (hypanthium) bearing 5 long bristles at the apex.
Small Scabious occurs on dry banks and pastures and prefers chalky soil. The plant is perennial with a woody rootstock.
Below: Field Scabious or Small Scabious?
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