By Dr. Laurent Legendre
Email : firstname.lastname@example.org
Pinguicula can be found in north, central and south America, in Greenland, in most parts of Europe, at the very north-western tip of Africa and in some scattered areas in Asia. They are not present in many parts of Asia, the middle east, most parts of Africa and southern Asia including Oceania.
The two main centres of diversity are found in central America/Caribbean and in Europe. North and east Siberia as well as far north east Asia are home to a small group of species which are either endemic (P. algida, P. ramose and P. variegata) or widespread (P. villosa, P. alpina, P. macroceras and P. vulgaris). The Himalaya in southern Asia is also hosting P. alpina. Large stretches of land in Canada and the north eastern and western coats of the USA are colonised by 3 species that can also be found in Europe or Asia (P. vulgaris, P. villosa and P. macroceras). The south eastern tip of the USA is also host to a group of related and endemic species. A few additional and endemic species are scattered on the slopes of the Andes mountains of south America from Columbia to Terra del Fuego in Argentina.
Even though Pinguicula can be found on many continents, most species have very restricted distribution ranges and can actually exist as just a few isolated colonies. On the same way, plant density may be high on a limited surface of land but individual colonies may be separated by large distances. Few species make exception to this rule. P. vulgaris has the largest distribution range, covering most of northern America, Greenland, Iceland, many parts of Europe and north western Russia. A closely related species, P. macroceras, covers the coats of north eastern Asia and north western America. P. villosa can be found on most of the arctic circle (except in western Canada). Finally P. alpina can be found in several parts of Europe, the Himalaya, north western Russia and northern Siberia. Species with mid-size distribution ranges include P. lusitanica and P. grandiflora in western Europe.
Most Pinguicula species originate from cold to very cold habitats at high latitudes or altitudes. Only the south eastern USA and Caribbean species live under warm climatic conditions. P. lusitanica on the western coast of Europe, P. crystallina on the northern part of the Mediterranean ocean and some Mexican species will be intermediate in their temperature requirements since they will need non-freezing winters and will appreciate warmer conditions during the summer. The only climatic conditions that are common to all Pinguicula are a very humid air, air temperatures that do not change too fast and a very wet soil during their growing season (often with flowing rather than standing water).
Most Pinguicula will be found growing in calcareous and rocky soils. However, some species are found in acidic bogs (P. lusitanica, P. corsica), some in sphagnum moss (P. villosa) and some on tree branches (P. casabitoana, P. lignicola).
Previously published groupings of Pinguicula species distinguished species on their ability to form an hibernaculum in the winter (temperate versus tropical types). The species of each type were subsequently divided into two subgroups on their ability to produce different leaves early in the spring and in the summer (homophyllous versus heterophyllous). Recent genetic studies do not totally support such views. Nevertheless, such clustering of species translates well into individual growing requirements.
The rosette of temperate species (homophyllous and heterophyllous) reduces to a tight bud made of scale-like non-carnivorous leaves in the winter. Such buds bear no roots with the exception of P. alpina (Asian species still need to be examined for this criterion). They should all be kept near or below freezing temperatures as long as they are formed. They indeed need to stay cold for a certain time before being able to sprout in response to higher temperatures. They are also sensitive to rotting if in contact with a non-frozen damp organic substrate.
Tropical species can either form carnivorous leaves all year long (homophyllous group) or develop a more or less compact winter bud made of succulent, non-carnivorous leaves (heterophyllous group).
Most tropical homophyllous species experience cold, even near, or periodically below freezing, temperatures in the winter. However, such cold periods do not last long and are quickly replaced by warmer conditions. Rosette sizes rarely diminish during such times. Growth may simply cease to resume as soon as temperatures warm up. Plants keep their roots all through the winter season and need to be kept wet all year long. Prolonged near-freezing conditions never result in healthy individuals.
Tropical heterophyllous species only form carnivorous leaves in the warm seasons. In the winter, they form more or less tight hibernacula made of succulent leaves. Winter hibernacula contain few or no root. They are very sensitive to rotting and should be kept dry until they resume growth. They should also be kept cold (below 10°C) and can stand below freezing temperatures is their substrate is totally dry.
Carnivorous Pinguicula leaves exhibit many tiny glands on their surface to glue, digest and eat insects and plant debris that are small enough not to escape. After catching a prey, the leaf rolls slowly around its catch to form a bowl below it or increase contact.
Pinguicula leaves harbour 2 types of glands: pedunculate glands that bear a drop of mucilaginous secretion (these are the tiny ones we see on the surface of the leaves and that generate rainbows when the sun shines on them), and sessile glands that lay flush on the leaf surface. The peduncles are made of only one cell and are therefore very small. The sessile glands do not generate large enough bodies of liquids to be seen by the human eye. For most species, the glands are only present on the upper surface of the leaves. But, in a few examples, they can be found on both sides of the leaves (P. gigantea, P. longifolia subsp longifolia). In that later case, the leaves are permanently erect or hanging in the air to prevent contact of the carnivorous underside of the leaves with the substrate. The density of glands on a leaf surface is purely related to the species and not to the state of feeding of the plant.
The pedunculate glands are involved in mucilage production (to glue preys). They secrete few digestives enzymes that are mostly produced by the non-mucilaginous sessile glands. Upon prey capture, the movement of the prey will trigger a large secretion of fluid from the pedunculate gland. This water comes from a reservoir cell located below the peduncle. As the reservoir empties itself, the gland sinks in the epidermis, therefore creating the curling effect. Together, bowl-formation and the mucilage will prevent the digested matter from escaping before being taken up. The small enzymatic activity of the early secretion will initiate prey digestion. The first nitrogenous compounds leaking from the digested prey will trigger a large enzyme production by the sessile gland. This secretory system is a ‘one-off’ system that will fire itself completely in one go and will not function again a second time if a second prey were to fall on the same spot.
Secretion and absorption through the glands is made possible thanks to the presence of holes (called cuticular gaps) in the cuticle (protective wax cover of the leaf) on top of the glands. These holes are like wounds through which the plant ‘bleeds’ (hence the droplets we see) permanently, placing its interior in direct contact with the outside world. Such cuticular holes are very rare for plants since they lead to dehydration and represent entry gates for infectious organisms. Luckily for Pinguicula, the digestive secretions prevent infectious organisms from developing. However, dehydration is a problem for Pinguicula which dehydrate faster than normal plants and will only perform best in very humid environments when they express carnivorous leaves. This phenomenon is worsened by the weak root system of Pinguicula which cannot pump much water quickly. In their non-carnivorous state (winter bud of most species), drier conditions can be supported. The lack of roots of winter resting plants will still require a fairly humid air.
Laws of physics reveal that water vapour pressure at a given relative humidity value (RH) increases exponentially as the temperature is raised. Lower exponential curves are obtained for lower relative humidity values so that the curves are very close to each other at low temperatures and far from each other at high temperatures.
Fig. 2 : RH versus Temperatures
Fig 3 : Psychrometric charts
Plant leaves contain small internal breathing air spaces (stomatal cavities) in which air relative humidity is always close to 100%. If the outside air has a lower relative humidity and the same temperature, a water vapour pressure deficit equal to the difference between the water vapour pressures at the two relative humidity values is created. This will lead to an efflux of water vapour from the plant interior to the outside (dehydration of the leaf) to re-equilibrate the pressures on both sides of the leaf. The closing of the breathing spaces is the normal response of a normal plant. For Pinguicula, this is of limited interest since dehydration can also occur through the cuticular gaps of the glands. Therefore, they are more prone to dehydration. Because of the above laws of physics, the dehydration of a Pinguicula will occur faster at high temperature than at low temperature if kept in environments that have similar relative humidities (let’s say 70%). This may explain why most Pinguicula species do better at colder temperatures, in the wild and in cultivation. Growing these plants at higher temperature will require rising the air humidity accordingly.
The above laws of physics also reveal that the sudden lowering of air temperature will lead to leaf dehydration even of the air is saturated in humidity. In this scenario, the plant is warmer than its environment. This will create a water vapour pressure deficit between the plant and its environment because water vapour pressures change with temperature. This dehydration effect will be more pronounced if the outside air is dry, if the original temperature is high or if the drop in temperature is severe. This may explain why Pinguicula in their native habitat and in cultivation do best in an environment where temperature does not fluctuate too fast or too much during the day. The curling of leaves or hiding behind grasses are things Pinguicula naturally do and that will lead to a buffering of external changes.