Yellow Asteraceae

Picris hieracioides (Hawkweed Oxtongue)

A large group of Asteraceae have yellow flowers and may be dismissed as 'types of dandelion' by the uninitiated. Determining the different species can be tricky, some for the beginner with whilst those species consisting of many 'microspecies', such as Taraxacum and Hieraceum can be difficult for all. Here we look at some of the easier types.

Yellow flowers are generally attractive to a range of insect pollinators, particularly hoverflies and also bees. Many insects are poorly able to see the color red but can see further into the blue spectrum than humans. Flowers exploit this by containing ultraviolet (UV) reflecting and/or UV-absorbing pigments. A yellow petal that also reflects UV light may appear purple to a bee. Bee-pollinated yellow flowers also tend to have UV-absorbing pigments towards the center of the flower-head which serve as nectar guides to direct the bee towards the nectar. To humans, the flowers simply look yellow!

Bird-pollinated flowers are also often yellow. In both cases, the yellow pigments are flavonoids in the sap of the main vacuole of each cell.

Picris hieracioides (Hawkweed Oxtongue)

Picris hieracioides (Hawk-weed Oxtongue) is a biennial with a preference for chalky soils and also found on field-borders and waste ground. The stem is covered in bristly hairs that are hooked at their ends. The flower-bearing peduncle is slightly thickened beneath each flower-head or anthode.  Recall that each 'flower' is really a cluster or flower-head (capitulum) of tiny differentiated flowers or florets and hence is an inflorescence, thus the flower-stalk is really a peduncle (inflorescence stalk) rather than a pedicel (the stalk of an individual flower). The stem-leaves are strap-shaped, the upper being lanceolate (shaped like a lance-head, i.e. narrow and pointed) with a wavy slightly undulating margin (repand) or sinuate. The upper stem-leaves are also semi-amplexicaul, meaning their bases partially encircle the stem. The radical leaves (growing from the base or rootstock) are linear (with straight parallel sides) or oblanceolate (lance-shaped but with the thinner end at the base, i.e. lanceolate but the other way around).

The pericline (the flask or cup covered with tiny leaf-like bracts called phyllaries) is oblong-ovoid when in bud and more conical after flowering. The outer phyllaries spread outwards and may recurve backwards whilst the inner phyllaries form a cup.  The phyllaries are covered in downy hairs or cilia and their bases are 'hispid', that is covered in hooked hairs that are stiff and feel rough. It closes again to form the conical flask-shape after flowering and then opens again to disperse the many fruit, each of which is a tiny single-seeded achene (though some use different terminology) with a pappus of white hairs to aid flotation and dispersal by the wind. The pappus is deciduous, that is cast off once it has served its purpose. All these stages are visible in the photo above.

The flower heads, each borne on its own branch, are grouped into a corymb, that is the branches grow so as to hold the flower-heads more-or-less at the same horizontal level. In one natural variety the flower-heads are borne on branches radiating from a main stem like the spokes of an umbrella to form a more compact umbel-like corymb.

Above: a fruiting capitulum in a closed state. The pappus hairs of the enclosed achenes just visible at the apex.

Picris echioides (Helminthotheca echioides, Bristly Oxtongue)

Picris echioides has many similarities to Picris hieracioides, but all the leaves bear white warts that bear prickly bristles and the 3 to 5 outer phyllaries are distinctively large and upright and almost enclose the inner phyllaries. These outer phyllaries are ovate (oval with the widest point nearer the base) and acuminate (the apex extending out into a point) with cordate (heart-shaped) bases. Their margins are covered in spiny bristles.

Picris echioides is a biennial and is found in waste places, roadsides and cultivated fields. The upright stems are covered in hooked hairs. The radical leaves form a flat rosette. Radical and lower stem leaves are oblong to oblanceolate in contour. The upper stem leaves are lanceolate and amplexicaul (their bases surround the circumference of the stem).

Another key difference between P. echioides and P. hieracioides is in the achenes: those of P. echioides have the pappus borne on a long narrow protuberance or beak that is longer than the main body of the achene. The function of this beak is unknown, but I suspect that it correlates with a deciduous pappus that eventually detaches.

Crepis vesicaria (Beaked Hawk's-beard)

Crepis vesicaria is native to central and norther continental Europe and parts of North Africa but introduced and naturalized in Poland, the British Isles, Australia and parts of North America (https://powo.science.kew.org/taxon/200365-1).

One of the key characteristics of Crepis vesicaria are the reddish stripes on the underside of the ligules (petals) of the outer florets, though such coloration also occurs in some other genera and so is not sufficient by itself.

The stems are about 15 to 60 cm tall.

The anthodes (capitula), each on the end of a branch forms a corymb. The peduncles are not thickened beneath the anthodes. Each achene has a pappus borne on a beak.

The phyllaries of the pericline are more-or-less hairy with greyish hairs. The inner phyllaries become slightly hardened in the fruit.

This species prefers chalky soils and is found on roadsides and in waste places in the British Isles. They have, for a long time, been reported as plentiful on the chalky soils of Kent.

The leaves are divided into toothlike lobes that are directed towards the leaf base. The leaves and stems are hairy.

Crepis biennis (Rough Hawk's-beard).

Many Asteraceae (and many Brassicaceae) produce yellow flowers. Many bee and bird-pollinated flowers are yellow. The cells of petals can contain water-soluble pigments in their sap of their large central vacuoles and/or lipid-soluble pigments in their plastids. Plastids may contain green chlorophyll (chloroplasts) or predominantly carotenoids which produce oranges (carotene) and yellows (xanthophylls). Flavonoids in the vacuoles (including anthocyanins) produce all purple and blue coloration  but also most reds. Combinations of pigments can produce additional colors.

In bee-pollinated flowers, like those shown here, the center of the petal contains flavonoids and is generally UV-absorbing, forming a central 'bull's-eye' to bees that are able to see UV (ultraviolet). The rest of the petal is UV-reflecting, containing carotenoids. The bee lands on the petals and moves towards the center. Once it encounters the UV-absorbing pigment it begins probing with its proboscis.

Hypochaeris radicata (Cat's-ear)

Hypochoeris (Hypochaeris) radicata (Cat's Ear) is a perennial found in fields, pastures and waste places in the British Isles. It is largely a native of the British Isles, but introduced and naturalized in other places (https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:225575-1).

The leaves form a rosette and vary in contour from oblong to oblanceolate to lanceolate and have wavy margins and are divided up into lobes. The leaves are usually roughly hairy. The numerous stems are scape-like, emerging from the rootstock and erect or ascending (curving upwards) to bear the capitula and are either leafless or bear only one or two small leaves. In addition there are small scale-like bracts beneath the capitulum.

The ligules are bright yellow, with the outer ligules being greyish (or brown-bronze) beneath. The phyllaries bear cilia (distinct hairs) on their midribs. The achenes are more-or-less beaked, with longer beaks in the achenes produced by the central florets. Each floret has a scale-like bract (receptacle scale or pale) beneath it which subtends the achene and is narrow and pointed.

Taraxacum (Dandelion) and the complexities of apomixis!

Taraxacum is well-known to many as the familiar Dandelion, though to the casual observer many yellow Asteraceae may be misidentified as 'dandelions'. Distinctive features include the soft, hollow scape (an inflorescence stalk coming directly from the rootstock) which readily exudes milky latex when broken and bears a single flower-head; and the basal rosette of toothlike leaves. However, this glosses over considerable taxonomic complexity, to say the least! Note that all the florets are ligulate (each having a straplike extension or ligule of the petal-tube) with each ligule having five points at the tip indicating that all five petals have merged and contributed to its development. All the florets have male and female parts.

Taraxacum is an example of a genus exhibiting the phenomenon of apomixis. At its simplest, in botany apomixis is parthenogenetic reproduction from seed without fertilization of the ovum. In other words it is the asexual development of seeds. (A related mechanism, also called apomixis occurs in some animals).

For the following explanation recall the following basic definitions in biology. A diploid (2n) cell has the normal complement of adult chromosomes, i.e. two sets, one paternal and one maternal. During sexual reproduction a cell undergoes meiosis, a reduction division, in which the chromosomes are first duplicated and then the cell divides twice into 4 daughter cells, each now having only a single set of chromosomes or half the adult complement; the cells are haploid (n). Some of these chromosomes will largely be paternal, some maternal but each will generally contain a mixture of paternal and maternal DNA due to crossing-over or exchange of DNA between each paternal/maternal chromosome pair. Polyploid cells and organisms contain additional sets of chromosomes, triploid contains 3 sets, tetraploid 4 and so on. During fertilization the male haploid gamete or sperm fuses with the female haploid gamete or egg cell to produce a diploid zygote which can then develop normally into a diploid adult. In this way sexual reproduction produces offspring with new combinations of genes by the three processes just described:

1. Random assortment of paternal and maternal chromosomes when one complete set of chromosomes is allocated to the gamete during meiosis.
2. Crossing-over or DNA exchange between corresponding paternal and maternal chromosome pairs during crossing-over.
3. Random and natural selection of the gamete that successfully fertilizes the egg cell.

Apomixis literally means 'without mixis' and mixis literally means 'mixes' and can be taken to refer to the normal mixing of genes that occurs during fertilization. Thus, originally it referred to asexual reproduction in general, but has since been restricted to refer to reproduction from seed without fertilization.

There are different types of apomixis occurring via different mechanisms in the various plant groups that exhibit the phenomenon and use of terminology may differ. First of all we have to remember that in angiosperms (flowering plants) double fertilization is the norm: two sperms are needed, one to fertilize the haploid ovum (egg cell) to create a diploid zygote and the other to fertilize the endosperm progenitor (central cell or polar nuclei) to form a triploid cell that divides to produce the endosperm that nourishes the embryo / early germling. In pseudogamy, pollination is still required for seed development but only the endosperm progenitor is fertilized, in other cases the endosperm will develop autonomously without being fertilized.

What about the egg cell? This may develop from non-disjunction or 'failed meiosis' in which the normal reduction division that normally gives rise to 4 haploid daughter cells fails and 2 diploid daughter cells are produced instead. Such an egg cell can then develop into an embryo asexually, without being fertilized. In this case the embryo will be a genetic clone of the mother, the mechanisms that introduce new assortments of genes into the offspring have been circumvented.

In Taraxacum, some adults are triploid apomicts. Since each cell contains 3 sets of chromosomes the normal mechanism of meiosis, which relies on chromosomes occurring in pairs (i.e. on an even number of chromosome sets) generally can not occur. Non-disjunction is the norm here, resulting in a triploid egg cell which then produces an embryo by parthenogenesis without being fertilized. Dandelions are autonomous apomicts so the endosperm progenitor also does not require fertilization to produce the endosperm.

However, it is not as simple as stating that 'meiosis can not occur' in a triploid, it can, albeit with somewhat unpredictable results. The fact that the egg cell is not produced by a reductive division is under genetic control. Indeed, Taraxacum still produces pollen. Pollen normally contains two sperm cells and meiosis is not blocked during pollen development in Taraxacum, so how are the three chromosome sets allocated to the pollen? Some pollen grains receive two full sets of chromosomes and are diploid, some receive one set and are haploid, and some receive one and a bit sets and are aneuploid. Additionally, some populations of Taraxacum contain diploid adults that reproduce via the normal sexual process: some populations are entirely sexual, some entirely apomictic and some contain a mixture of sexual diploids and apomictic triploids. The complicated life-cycle of Taraxacum is represented in the diagram of chromosome flow below, in which S indicates one full set of chromosomes:

What is going on here? I have used S to represent a chromosome set rather than n. The reason being that many plants are normally polyploid, so a tetraploid plant (4n) may reproduce by normal sexual means to produce diploid gametes (2n). However, since tetraploidy is the norm for such a plant we may still regard the parent as diploid and the gametes as haploid, in which case n gets confusing and we usually use x. So, a tetraploid plant is 4x = 2n and its gametes are 2x = n, where x is one full set of ancestral chromosomes but the plant now behaves as if one full set is two ancestral sets (n = 2x). However x may be confused with X, the female sex chromosome in a diagram, so instead of x I have used S to donate one full set of ancestral chromosomes. A rectangle represents an adult, a circle a female gamete (egg cell) and a hexagon a male gamete (inside the pollen grain). The green arrows represent meiosis.

This type of apomixis, which is ane xample of entopic apomixis since the embryo develops in the usual place, occurs in Taraxacum in which the megaspore undergoes a failed meiosis to produce unreduced (3S) nuclei. In ectopic apomixis additional megaspores develop from cells in the nucellus (nutritive tissue enclosing the embryo sac) that produce embryo sacs in addition to the usual haploid and sexual embryo sac. In somatic apomixis, both sexual and apomictic embryos may be produced. This occurs for example in Potentilla of the rose family (Rosaceae).

In addition, apomixis can be divided into two other broad types. In the first type, the embryo whether derived in the usual location or ectopically, develops into a morphological gametophyte, that is an embryo sac (containing the egg cell) which is the type so far described, but since the cells of the embryo sac are not reduced to the haploid state (n) it is misleading to refer to at as a gametophyte and it would be better called a pseudogametophyte but it does still function as an embryo sac to contain the developing embryo.  Thus, I refer to this type of apomixis as pseudogametophytic apomixis. Recall that this can be entopic or ectopic. Alternatively, in some species, apomixis takes place when a cell, entopically or ectopically, develops directly into an embryo without passing through an embryo sac stage. This could be called sporophytic apomixis, since the sporophyte is the parent plant and the embryo develops directly from its tissues. What we have in Taraxacum is pseudogametophytic entopic apomixis with failed meiosis.

When multiple embryos develop in the same ovule, by whatever means, it is called polyembryony. Many of the additional terms used to describe apomixis are misleading and are not mentioned here and different authors use the terms differently though for historic reasons current usage seems inappropriate to me.

So why does Taraxacum produce pollen?

The answer is in the diagram. The normal sexual cycle on the left and the asexual apomictic cycle on the right can be clearly seen, but note that the two are connected. Haploid (S) pollen from an apomict can fertilize a normal sexual egg cell to produce a sexual diploid, but note also that 'diploid' pollen (SS) can also fertilize the egg cells of a sexual plant to produce new apomictic clones (SSS). Thus, sexual reproduction allows the establishment of new apomictic types. This could be particularly important since asexual reproduction could potentially lead to the accumulation of mutations within a clone over the generations, perhaps adversely affecting fitness, but occasional sexual reproduction could reinvigorate the population with an influx of new genes, generating new clones. On the other hand, asexual reproduction is advantageous when a clone is well-adapted to its environment and when the environment is too harsh or otherwise unsuitable for sexual reproduction.

Mixed sexual and apomictic populations occur in central Europe as far North as the Netherlands, populations further North are entirely apomictic. In the British Isles alone the various clones fall into over 200 recognized types. These types were at one time considered varieties of Taraxacum officinale, but are now regarded as microspecies grouped into sections of Taraxacum, sometimes lossely lumped together as Taraxacum officinale s.l. (s.l. = sensu lato, that is 'in the loose sense'). I am not convinced that there is a good way to treat these using binomial nomenclature. The various sections of Taraxacum can be defined morphologically using such characters as leaf shape (especially how divided the leaves are), plant size, achene size and color and color of the stripe on the underside of the ligule.

Pilosella

Article created: 13 May 2023.