Introduction

The fact that low summer temperatures restrict the distribution of plants is an everyday experience. Gardeners who try to grow exotic species find that they will not set ripe fruits except in unusually warm summers. After unusually cold summers buds and fruits may fail to ripen and are damaged by early frost. Such early frosts have sometimes destroyed the grain harvest over large areas, resulting in famine. By comparing the latitudinal distribution of cultivated species in Europe a characteristic pattern of distribution limits is found towards the north. Some species are restricted to the southernmost parts of the area with the warmest summers. Typical examples are the traditional cultivation of grapes, going north to the Rhine Valley in Germany, into southern England but not reaching northern England; and the cultivation of maize north to central England and southern Sweden, of wheat north to Scotland and in the valleys of southern Scandinavia north to Trondheim in Central Norway, and potatoes which can be grown all over the British Isles and north to the inner fjords of North Norway.

Summer temperatures limit the distribution of species altitudinally as well as latitudinally. The altitudinal limits of plants in the Alps tend to be higher than in Fennoscandia. This is shown in Fig. 21, where the altitudinal limits of plants in southern Norway are plotted against the same species in the Alps. It is seen that the altitudinal limits in the Alps are, on average, 1000 m higher than in southern Norway.

Northern Europe is situated in the low pressure belt between the subtropical high and the polar high. In areas of the Atlantic Ocean that are south and southeast of Iceland the average atmospheric pressure is low and this directs southwesterly winds to Northern Europe. These southwesterly winds bring heat to northern latitudes in western Europe. Warm water transported by the Gulf Stream and North Atlantic Drift also contributes to this. Thus North Europe has, for its latitude, an unusually mild climate. At the same latitude as southern Norway but west of the Atlantic is southern Greenland, where most of the land is covered by an inland ice sheet.

In this climatic system, cyclones are generated that travel eastwards with rain that is enhanced when the cyclones meet the mountains of Scotland, Scandinavia and the Alps. The cyclones have a more northerly position in summer than in winter and the Mediterranean areas thus have drought during summer. In Northern Europe there is cyclonic rain in all the seasons, with a maximum in late summer and autumn. The other major source of precipitation is convective rain, especially during the warm season. On hot days the air near the ground is heated and becomes unstable so that warm air rises up through the atmosphere, is cooled, and liberates precipitation, often as thunderstorms. This is the most important source of rain in the eastern, more continental parts of Europe.

The boreal element in Europe is a northeastern element centred on the great conifer formation, the taiga, of northern Russia and Siberia. Boreal species form a series of equiformal progressive areas in the sense of Hultán (1937), from species with a very restricted distribution in northern Russia and perhaps penetrating into northern Finland to species with a much wider distribution, reaching the northern parts of the British Isles and the higher mountains of the Iberian Peninsula and the Balkans. It is typically a forest element. Only a few species extend north of the arctic forest-limit or much above the altitudinal timber-line.

The distribution patterns show that boreal species tend to be absent from areas with mild winters along the southwestern lowlands of Europe. Such species were called ‘southwest coast avoiders’ by Conolly & Dahl (1970) (see also Dahl 1951). They are able to tolerate high summer temperatures as shown by their occurrence in the lowlands of eastern and Central Europe. In this respect they differ from the true arcticalpine plants. However, in the southwest they are restricted to higher elevations.

Climatic correlations

Many of the boreal species have distribution patterns that are the inverse of the atlantic species. For example, the horizontal distributions of Picea abie and Ilex aquifolium overlap only in the former Yugoslavia, but there Picea grows in the mountains whereas Ilex is restricted to lower elevations. Another example from Norway, pointed out by Blytt (1869), is that only in a restricted area along the Sognefjord does the atlantic species Digitalis purpurea occur in the same area as the boreal Aconitum septentrionale.

The atlantic and oceanic elements consist of species with a southern and western distribution in Europe. The definitions of these elements have been thoroughly discussed by, for example, Troll (1925), Holmboe (1925), Kotilainen (1933), Degelius (1935), Fægri (1960), Ratcliffe (1968) and StØrmer (1969).

In dealing with the atlantic and oceanic elements it is important to distinguish between stenohydric and poikilohydric plants. Almost all vascular plants are stenohydric; if their cells dry out they die. Lichens and bryophytes are poikilohydric, as their cells quite normally dry out without causing serious damage. The ecology and distribution patterns of stenohydric and poikilohydric plants are quite different and to emphasise these differences, I will use the term atlantic for stenohydric plants and oceanic for the poikilohydric plants.

The atlantic element

Climatic correlations

An isotherm map of the temperature in the coldest month, calculated for the lowest point in each square grid in Atlas Florae Europaeae,is given in Fig. 11. The distribution patterns of atlantic plants are correlated with these isotherms.

Within the atlantic element a number of sub-elements and groups can be recognised according to the correlations between the distribution limits of the species and the winter temperature isotherms. The following sub-elements can be recognised (see Appendix II which lists all the correlations).

The British–Mediterranean sub-element

This consists of species limited in Britain and Ireland to areas with winter temperatures higher than +6C. Isoetes histrix (Fig. 12) is an example.

Eilif Dahl had one of the most original and creative minds in plant geography, as well as in plant sociology and mountain ecology. His approach to plant geography went far beyond the description of distribution patterns and the establishment of correlations between distributions and particular climatic variables. He always strove to try to understand what the underlying physiological mechanisms were that influenced and controlled the observed distributional patterns. He proposed that many mountain plants are restricted to high elevations because of their intolerance to high maximum summer temperatures in the lowlands. He suggested that thermophilous species are restricted in their range by temperature dependent ATP production during dark respiration. He hypothesised that many eastern boreal species are absent from western oceanic areas because of the inability of the species to cope with alternating mild and cold periods in winter and spring. He also strongly championed the importance of history, particularly glacial survival on ice-free refugia (nunataks) of arctic plants during the glacial stages, to explain the present-day distributions of several high-arctic species. Throughout his plant geographical studies Dahl brought to bear his wide range of scientific interests and abilities in, for example, physics, chemistry, geology, climatology, meteorology and mathematics, as well as plant ecology and plant systematics.

In 1986 Eilif Dahl told me he was planning to write a book on the plant geography of Northern Europe. I was naturally excited by the idea of a book by Eilif Dahl that would bring together his life's work and ideas on this fascinating topic.

Plants that benefit from human activities and thereby increase their population size or geographical area are termed synanthropic or hemerophilous. This includes weeds in fields or gardens, also called agrestals or segetals, and ruderals that occupy areas where human activity has disturbed the natural vegetation in abandoned plots, along roads or railways, etc. Weeds are mostly annual, whereas ruderals are mostly perennial.

Phytogeographical problems of anthropochorous plants have been treated by Linkola (1916, 1921), Jessen & Lind (1923), Salisbury (1961), Berglund (1966a, b), Godwin (1975), Holzner & Numata (1982), Mucina et al. (1984), Willerding (1986), Di Castri et al. (1987), Kornek & Sukopp (1988) and Sukopp & Hejny (1990). The role of man in European vegetation history has been summarised by Behre (1988).

Before the advent of agriculture in the Neolithic period man lived as a hunter-gatherer and his impact on the flora was different from animals in only a few ways. He contributed to the dispersal of diaspores and no doubt encouraged growth of some species around his habitations as some species benefit from the addition of nitrogen and phosphorus to the soils. The use of fire may also have affected the vegetation, but we know little about this.

With the advent of agriculture, the effects became more important. The first agriculturalists used fire as a means to clear the vegetation. Trees were felled with stone axes and the slash thus produced was burnt. The previous vegetation was thus destroyed and the ash enriched the soils with plant nutrients.

Arctic plants have their main distribution north of the arctic timber-line. Alpine plants have their main distribution above the climatic alpine timber-line. Montane plants have their main distribution in high-lying forests in the southern part of their area but can occur at low elevations in the north. Their lower distribution limits fall from the south towards the north and generally from continental to oceanic areas. Many otherwise alpine species can be found at sea level along the west coasts of the British Isles and Fennoscandia.

The alpine and montane plants have a distribution approximately the inverse of the thermophilic plants. Since the upper and northern limits of the thermophilic plants are related to summer temperature, the lower and southern limits of the alpine plants must be negatively related to high summer temperatures. The question is, what are the physiological mechanisms responsible for these distribution patterns.

One obvious explanation is competition (Pigott 1978). Alpine and arctic plants are low-growing and hence unable to compete for light where climate is favourable for tree growth. This hypothesis can easily be tested by planting alpine species in botanic gardens in the absence of competition and observing whether they survive. Often they do quite well, but not always. Some species are hard to keep outdoors for many years in botanic gardens. Very often they survive the spring but seem to suffer in the summer heat. They have a better period in autumn and next spring, but after a few years they are so weakened that they die.

The ultimate challenge for future Volvox research is to visit the past and retrace the pathway that led to the present.

Those of us currently engaged in Volvox research are excited by the challenge of capitalizing on the technical advances described in Chapter 7 to elucidate the detailed mechanisms by which cellular differentiation is programmed and executed in modern V. carteri. But when this goal has been achieved, the most distinctive attributes of Volvox as a developmental-genetic model system will still remain to be exploited. Although Volvox does offer a superb opportunity for defining the genetic and molecular basis of a highly interesting form of dichotomous cellular differentiation, it is not this feature that most clearly distinguishes Volvox from slime molds, fruit flies, plants, nematodes, zebrafish, mice, and other model organisms currently under intensive developmental-genetic investigation. Rather, it is the potential that it offers for tracing out in some detail the pathway by which such a program for cellular differentiation evolved.

As discussed in Chapter 1, multicellular organisms with a capacity for cellular differentiation clearly have evolved from simple unicellular ancestors numerous times; hence molecular mechanisms leading to dichotomous differentiation must have been independently invented many times in the past.

But in every other group that has been examined thus far, such inventions are buried so deep in antiquity that details of the pathway leading from unicellularity to multicellularity almost certainly have long since been obliterated by tectonic forces and the shifting sands of genetic drift.

There is a widespread belief that “developmental constraints” serve as boundary conditions that limit the types of morphological innovations that can arise within any group of organisms (Alberch 1982; Maynard Smith et al. 1985), though there is less than universal agreement about how developmental constraints are to be defined and recognized. The basic concept is that the range of morphological innovations that can be generated within any group of organisms is constrained by certain fundamental features of the cellular organization and developmental biology of that group of organisms.

The hypothesis to be developed in this chapter is that the extraordinary degree to which organelles within a green flagellate cell are interconnected by a highly regular cytoskeletal network, as outlined in the foregoing quotation, constitutes a fundamentally important developmental constraint that (when combined with the presence of a coherent cell wall and the selective pressures discussed in the preceding chapter) led, with a certain degree of inevitability, to the appearance of organisms, like Volvox, with a division of labor between somatic and reproductive cells.

With those words summarizing his observations of several interesting spontaneous mutants of V. carteri, Richard Starr tried to direct the attention of those assembled for an annual meeting of the Society for Developmental Biology toward a promising new avenue for analyzing the genetic control of cell differentiation. But for some reason that was not an avenue destined to become quickly crowded with fellow travelers. Over the next quarter century, only nine laboratories would publish one or more studies involving the use of V. carteri mutants, and of those, only three would make any sustained efforts to use genetics as a tool for dissecting V. carteri development. Despite that lukewarm response to Starr's summons, his perception that V. carteri had substantial promise as a developmental genetic system was soon reinforced, principally through the efforts of one laboratory.

Robert Huskey, formerly a bacteriophage geneticist, was the first to join Starr in the exploration of V. carteri developmental genetics.

The foregoing statement is as valid now as it was when it was written 27 years ago. Modern studies of Volvox biology began in Richard Starr's laboratory in the 1960s, when William Darden, then a graduate student, demonstrated that axenic cultures of Volvox aureus could be maintained indefinitely in a chemically defined medium that had been developed a few years earlier for culturing other types of algae (Provasoli & Pintner 1959). In such cultures he was able to observe and study all stages in the asexual and sexual life history of V. aureus under controlled conditions (Darden 1966). Later, he and his associates would use this culture system to examine a variety of aspects of V. aureus biology (Darden 1968, 1970, 1971, 1973a,b; 1980; Deason et al. 1969; Darden & Sayers 1969, 1971; Ely & Darden 1972; Tucker & Darden 1972).

As the foregoing quotation so colorfully indicates, Volvox often appears in great abundance within days after the warm rains of early summer accumulate in depressions in the ground, and it is often joined by several of its colonial relatives. The ability of these volvocaceans to appear as soon as vernal pools are formed (or as soon as temperate lakes have thawed and stabilized in the spring) depends on a feature that they share with Chlamydomonas and many other green algae: When conditions begin to deteriorate toward the end of the growing season, they switch from asexual to sexual reproduction and produce dormant zygotes, or “zygospores,” that are resistant to desiccation and freezing (Coleman 1983). These zygospores settle into the mud to wait out the adverse times; then, once favorable conditions return, they quickly germinate, and the germlings swim toward the surface and begin proliferating asexually.

Many volvocaceans (like many other algae) are astonishingly cosmopolitan, being found in similar environments around the world. For example, isolates of Gonium pectorale from Europe, Asia, and all parts of North America have been shown to be members of an interfertile population that is capable of sharing a single gene pool and is accordingly quite homogeneous at the DNA-sequence level (Stein 1966a,b; Coleman et al. 1994). Even “species” such as Pandorina morum that are extensively subdivided into reproductively isolated units, or syngens, are actually cosmopolitan, because members of a single syngen can be found on several different continents (see Chapter 2).

When he was writing his magnum opus, it is unlikely that August Weismann had any notion just how gradual the “approach to the truth” regarding the role of the hereditary substance in embryonic development would be in the next eight decades – or how different the outcome would be from the one that he was predicting! On the other hand, it is equally unlikely that when Kochert published the first studies of Volvox nucleic acids (only three years after he had described development in the forma of Volvox carteri that carries Weismann's name) he could have anticipated how dramatically the pace – and indeed the very nature – of studies of “nucleic acid metabolism” was destined to change in the next two decades.