Foxtail Pine P. Four-Leaf Pinyon P. Two-Leaf Pinyon P. One-Leaf Pinyon P. Ponderosa Pine P. Coulter Pine P. Digger Pine P. Torrey Pine P. Jeffrey Pine P. Sugar Pine P. Another species left image called the Washoe Pine P. In addition, the Beach and Lodgepole Pines are now recognized as subspecies of P. According to R. Lanner Conifers of California , , there may be other significant changes in the pines of California. Allozyme studies in two-leaf pinyons Pinus edulis of the New York Mountains indicate that these populations are biochemically and genetically consistent with nearby one-leaf pinyon Pinus monophylla , and that P.
The unusual New York Mountains population appears to be a 2-needle variant of P. According to Lanner, the latter species has five needles per fascicle and occurs in San Diego County.
The hybrid hypothesis might explain the perplexing variation in needle number for P. Foxtail pines Pinus balfouriana on the 11, ft m slopes of Alta Peak. The 13, ft. Left: Seed cones of cypress Cupressus from groves in southern California.
Tecate cypress C. Sargent cypress C. Piute cypress C. Cuyamaca cypress C. Smooth-bark Arizona cypress C. Rough-bark Arizona cypress C. Right: Seed cones of cypress from groves in central and northern California. Monterey cypress C. Gowen cypress C. Santa Cruz cypress C. Mendocino cypress C. Macnab cypress C. Modoc cypress C. Male pollen cones of the Piute cypress Cupressus nevadensis [syn.
Each scalelike leaf bears a dorsal gland that exudes a resin droplet red arrow. Interior cypress species such as this one typically have glaucous, resinous foliage, presumably an adaptation to dry, arid habitats.
Foliage and pollen cones of the Smooth-bark Arizona cypress Cupressus glabra [Syn. Foliage of the Tecate cypress C. The scalelike leaves of Arizona cypress are glaucous and very glandular sticky.
The scalelike leaves of Tecate cypress are green and without dorsal resin glands. Right: Grove of Piute cypress C. The Piute cypress are more drought resistant, with gray glaucous , glandular resinous foliage similar to the Arizona cypress. In fact, some botanists now consider the Piute cypress to be a subspecies of the Arizona cypress and have named it C.
This species typically grows on outcrops of serpentine in the Coast Ranges of central and northern California. Serpentine is a shiny rock with a waxy luster and feel. It varies in color from creamy white and shades of green to black. In California, many species of rare and endangered plants are endemic to serpentine outcrops. Genetic drift has undoubtedly occured in isolated cypress groves such as this one, which are often referred to as "arboreal islands.
In most cases, these detailed phylogenies have revealed more complex evolutionary patterns than originally anticipated. In Amaranthaceae sensu stricto ss excluding Chenopodiaceae , for example, two lineages were hypothesized by Kellogg based on poorly resolved phylogenies.
Three lineages were suggested by Kadereit et al. Sage et al. A more problematic situation is present in the Chenopodiaceae ss. While recent phylogenetic work has clarified relationships within this family, patterns of C 4 evolution remain uncertain because the C 3 and C 4 pathways have not been clearly identified in some parts of the phylogeny. To clarify matters, the photosynthetic types were mapped on a phylogenetic tree for the Chenopodiaceae inferred from data accumulated in recent studies Fig.
This approach indicates that 10 C 4 lineages are present in the Chenopodiaceae ss. More lineages may be present, as there is a possibility that the Salsola kali and Halothamnus groups may represent two independent C 4 lines. The situation is also unclear in Camphorosmeae, where anatomical variations could be interpreted as the fingerprint of two different C 4 origins Kadereit and Freitag, The distribution of photosynthetic types also indicates a C 4 to C 3 reversion and reacquisition of the C 4 pathway in the branches between the S.
Kranz types and biochemical types are from Muhaidat et al. Grass lineage names follow those of Roalson and Christin et al. Flaveria brownii is physiologically a C 4 -like intermediate, in that it expresses Rubisco in the mesophyll Cheng et al. It is treated here as an independent C 4 clade as it has a fully functional C 4 cycle, and photosynthetic gas exchange properties and resource use efficiencies that are equivalent to those of many C 4 species.
The distribution of C 4 photosynthesis in the Amaranthaceae sensu stricto. The phylogeny was obtained through Bayesian inference on the trnK—matK data set of Sage et al. It is rooted on the Achatocarpaceae. Bayesian support values are indicated near branches. Clades that contain a single photosynthetic type for which names are available are compressed and coloured in red for C 4 , blue for C 3 —C 4 , and black for C 3.
Names of C 4 clades are in bold and numbers beside C 4 groups correspond to lineage number Table 1. The distribution of C 4 photosynthesis in the Chenopodiaceae sensu stricto. The phylogeny was obtained through Bayesian inferences on the nuclear internal transcribed spacer ITS and plastid psbB—psbH markers generated in previous studies Kapralov et al.
Clades that contain a single photosynthetic type are compressed and coloured in red for C 4 , blue for C 3 —C 4 , and black for C 3. Asterisks indicate single-celled C 4 taxa.
Subfamilies are circumscribed on the right. Species level resolution has also facilitated the identification of the centres of origin for many of the listed C 4 lineages. This can be accomplished by identifying closely related C 3 and C 4 species within a phylogeny, and any related C 3 —C 4 intermediate species.
By mapping the geographic distribution of the sister groups and intermediate forms, the region where the C 4 lineage arose can in many cases be identified with a good degree of confidence, thereby facilitating evaluation of the environmental conditions that promoted the emergence of C 4 photosynthesis. To visualize broadly the phylogenetic distribution of C 4 taxa, as many C 4 groups as possible were mapped onto a recently published phylogeny of angiosperms Smith et al.
Because the phylogeny was built for other purposes and taxon sampling was therefore agnostic with respect to the photosynthetic pathway, it was felt that this presents a valid means to evaluate broad phylogenetic patterns of C 4 evolution across angiosperms. It was possible to place 47 C 4 lineages on the tree.
In several cases e. The phylogenetic distribution of C 4 lineages in the angiosperms, depicted on a phylogeny of angiosperms species that was pruned from the viridiplantae phylogeny of Smith et al. C 4 lineages are indicated by red branches. Numbers beside named lineages refer to the estimate of the number of independent origins of C 4 in that clade.
Forty-seven of the 62 C 4 lineages could be placed on the phylogeny; in several cases, C 3 taxa were highlighted to represent the position of closely related C 4 species see text.
Table 1 lists 62 distinct groups of C 4 taxa in terrestrial and aquatic vascular plants. Some diatoms can also operate C 4 metabolic cycles but are not discussed here Reinfelder et al. These 62 groups are treated as distinct evolutionary lineages, on the assumption that each lineage arose from ancestors that were fully functional C 3 species.
It is recognized that the evolutionary independence of some C 4 lines could be debated if their common ancestors share well-developed traits associated with the C 4 syndrome, notably Kranz anatomy.
This could be the case in Flaveria , Mollugo , and Camphorosmeae, where multiple C 4 species may derive from well-developed C 3 —C 4 intermediates expressing Kranz-like anatomy McKown et al. None of these potential lineages are included in Table 1 because of phylogenetic ambiguity and uncertainty regarding evolutionary independence. Thirty-six of the 62 lineages occur in the eudicots, six in the sedges, and 18 in the grasses Table 1. The aquatic monocot family Hydrocharitaceae has two C 4 lineages, in the genera Hydrilla and Egeria.
C 4 photosynthesis in these two groups is distinct from that of the other 60 lineages where plants have aerial photosynthetic structures. In Hydrilla and Egeria , the C 4 pathway operates in submersed leaves and concentrates CO 2 from the cytosol into an adjacent chloroplast of a single cell Bowes, In all other known C 4 plants, the C 4 pathway concentrates CO 2 from a mesophyll-like compartment into a distinct inner tissue region 58 lineages or concentrates CO 2 from an outer to an inner region of the same cell in two Chenopodiaceae lineages, Bienertia and Suaeda aralocaspica ; Edwards and Voznesenskaya, Clustering is evident in the distribution of the lineages in the angiosperm phylogeny Fig.
Of these 15 lineages, the largest is Atriplex , with — C 4 species Kadereit et al. The most species-rich C 4 eudicot lineage other than Atriplex is Euphorbia section Chamaesyce with about species.
Fifteen C 4 species are known in the Eleocharis section Tenuissimae ss , while 21 C 4 species are known from Rhynchospora. The smallest C 4 sedge lineage is Eleocharis vivipara with a single C 4 species Bruhl and Wilson, ; Roalson et al.
Thirty-five to species are in each of the lineages represented by Altoparadisium , Aristida , Axonopus , Danthoniopsis , Digitaria , Echinochloa , Eriachne , Paspalum , and Stipagrostis. The estimates of C 4 grass numbers within numerous lineages will change, as many grass genera and species cannot be accurately placed in a lineage yet, due to limited phylogenetic information. This is especially true for Panicum , which contains several hundred C 4 taxa, but is highly polyphyletic Aliscioni et al.
NAD-malic enzyme ME is used by species from 20 lineages. Most of the lineages with NAD-ME species are eudicots, as only two grass and two sedge lineages include species that are classified into this biochemical subtype. Only grasses appear to utilize PEP carboxykinase PCK as the primary decarboxylating enzyme; however, this enzyme may also be active as a secondary decarboxylase in the C 4 cycle of eudicots in the Sesuvioideae Muhaidat et al.
In the grasses, four C 4 lineages include species that use PCK as the primary decarboxylating enzyme. Anatomical types are far more varied than biochemical subtypes. Some 22 Kranz anatomy types have been described, and numerous variations within a number of these subtypes are noted Brown, ; Dengler and Nelson, ; Kadereit et al.
In the eudicots, the most common anatomical type is the Atriplicoid, which occurs in at least 20 of the 36 eudicot lineages Table 1. The next most common Kranz type is the Salsaloid, occurring in three lineages in the Chenopodiaceae and one in Calligonum Polygonaceae. In sedges and grasses, the variation in Kranz type is greater, with most lineages having evolved a unique version of C 4 anatomy.
The classical type of Kranz anatomy is described for seven grass lineages; however, there can be important variations in the anatomies that are associated with biochemical subtypes Dengler and Nelson, Twenty-one distinct clades have been identified that contain species with photosynthetic characteristics that are intermediate between C 3 and C 4 species Table 2. Ten of the C 3 —C 4 groups branch immediately sister to C 4 lineages, which is consistent with models proposing that C 3 —C 4 intermediacy originated before C 4 photosynthesis and served as an ancestral stage Monson et al.
Some of these intermediates, however, do not appear at sister nodes. Where C 3 species branch between the C 3 —C 4 intermediate and a C 4 node, as occurs with the C 3 —C 4 intermediate Mollugo verticillata , it appears that the C 3 —C 4 line has independently arisen from different C 3 ancestors than the C 4 line Christin et al.
Where a C 3 —C 4 species branches between two C 4 nodes, as occurs with the C 3 —C 4 intermediate Portulaca cryptopetala Ocampo and Columbus, , a reversion from the C 4 condition is possible. Notably, about a quarter of the identified C 3 —C 4 species occur in taxa that are not closely related to any C 4 lineage.
These patterns highlight the need to consider the C 3 —C 4 condition as a distinct photosynthetic adaptation in its own right, and not just a transitional stage leading to the C 4 condition.
The postulated lineages of C 3 —C 4 intermediate photosynthesis in higher plants. C 3 —C 4 as defined here refers to photosynthetic modifications that include refixation of photorespiratory CO 2 in bundle sheath cells, and the engagement of a C 4 metabolic cycle. Compiled from Sage et al. Geographic centres of origins for C 4 photosynthesis can be estimated for most eudicot lineages, and some of the sedge and grass lineages Table 1. In the eudicots, lineages occur in one of six centres of origin, corresponding to regions of the Earth that are now warm, semi-arid, and arid Fig.
Central Asia, North America, and a region corresponding to northeast Africa and southern Arabia produced the most C 4 eudicot lineages, with 4—11 each. Two centres corresponding to semi-arid regions of South Africa and South America each produced 4—5 C 4 eudicot lineages, while the driest continent, Australia, produced only one C 4 lineage in the eudicots that can be confirmed at this time.
Locations for the centres of origin for 35 of the 36 C 4 eudicot lineages listed in Table 1. Numbers shown correspond to lineages listed in Table 1. Unlisted lineages have an unknown centre of origin. Identifying the geographic origins of the C 4 monocots is more problematic due to their wide geographical distribution and greater uncertainty regarding the phylogenetic placement of the C 4 lineages.
Two C 4 grass origins in Africa are apparent, in Centropodia and Alloteropsis. Only one C 4 grass clade Neurachne is known to have originated in Australia. Eleocharis vivipara is the only sedge lineage where a centre of origin in Florida, USA can be postulated at this time.
This compares with 45 lineages listed by Sage and 31 listed by Kellogg The increase in the number of lineages is largely due to improved phylogenetic coverage of clades that include C 4 plants, and a more complete accounting of C 3 and C 4 occurrence in the species within these clades. As an example, where only three clades were resolved in the Amaranthaceae ss in Kadereit et al.
Similarly, early molecular phylogenies suggested a minimum of four C 4 grass lineages Kellogg, , a number that has now increased to The current list of C 4 groups is most probably incomplete, as relationships in some clades are still unresolved. C 3 —C 4 intermediacy is a term originally used to describe plants with traits intermediate between C 3 and C 4 species, on the assumption that they might represent an evolutionary transition Kennedy and Laetsch, ; Monson et al.
Currently, C 3 —C 4 intermediacy mainly refers to plants with a photorespiratory CO 2 -concentrating mechanism, where expression of the photorespiratory enzyme glycine decarboxylase GDC is localized to BSCs Monson, ; Duvall et al.
Localization of GDC to the bundle sheath forces all the glycine produced by photorespiration to move into the BSCs to complete the photorespiratory cycle. Following the mutation leading to GDC localization, C 3 —C 4 species evolve many C 4 -like traits such as close vein spacing and enlarged BSCs to optimize the efficiency of photorespiratory CO 2 concentration Sage, While these developments may facilitate C 4 evolution Bauwe, , they also confer fitness in their own right, as reflected by numerous C 3 —C 4 lineages that are distinct from C 4 clades, and the ecological success of numerous C 3 —C 4 species in warm to hot environments Monson, ; Christin et al.
Twenty-one distinct lineages of C 3 —C 4 intermediate plants have been identified. The first C 3 —C 4 intermediate described was M. After this initial phase of discovery 25—35 years ago, the identification of new intermediates trailed off until recently, when phylogenies and isotopic screens helped identify additional intermediates. For example, the Euphorbiaceae phylogeny of Steinmann and Porter identified two species of Chamaesyce that are basal in this large C 4 group.
One of these, C. Intermediates have also been found recently in Cleome and Portulaca Voznesenskaya et al. Most of the known C 3 —C 4 intermediates are in eudicots, while only two intermediate lineages are described in the grasses, and one each in the Hydrocharitaceae and sedges.
This discrepancy may reflect greater species turnover in grasses and sedges, which led to a greater rate of extinction of C 3 —C 4 taxa. Alternatively, the greater number of eudicot intermediates may reflect sampling bias. Most known C 3 —C 4 intermediates are associated with C 4 eudicots, because investigators often focused on eudicot genera having both C 3 and C 4 species Monson, ; Vogan et al.
With a wider sampling and improved phylogenetic resolution of poorly studied groups, the tally of C 3 —C 4 lineages should grow in the near future. In order to detect potential C 3 —C 4 intermediates lacking a C 4 cycle, anatomical screens are a useful first step, but detailed physiological studies with live material will still be needed for confirmation. For the eudicots, and the handful of monocots where the centre of C 4 origin can be estimated with confidence, there appear to be six geographic regions where the C 4 pathway evolved.
All of these correspond to areas that are now semi-arid to arid, with summer precipitation from monsoon weather systems.
By identifying the putative centres of origin for many of the C 4 lineages, we hope to facilitate follow-up studies that will evaluate the environmental selection factors responsible for the evolution of specific C 4 lineages. Such studies could examine the ecophysiology of the close C 3 and C 3 —C 4 relatives of the C 4 lines currently present in the centres of origins.
Alternatively, paleontology studies could correlate past environmental events with the appearance of a C 4 lineage in a given area. To date, the leading environmental hypothesis for C 4 evolution is that reduction in atmospheric CO 2 in the late Oligocene increased photorespiration in warm climates, thereby facilitating selection for CO 2 -concentrating mechanisms such as C 4 photosynthesis Sage, , ; Christin et al.
However, C 4 photosynthesis repeatedly arose in the 25—30 million years since the late-Oligocene CO 2 reduction Christin et al. In light of this, it is better to think of low CO 2 as a pre-condition, or environmental facilitator, which acted in concert with multiple selection factors.
Other proposed drivers of C 4 evolution include increasing aridity, creation of high light habitats, increasing seasonality, fire, and large animal disturbance Sage, ; Osborne and Freckleton, ; Edwards and Smith, ; Osborne, While a careful paleo-evaluation is beyond the scope of this study, it should be noted that global climates became cooler and drier in the past 40 million years, promoting the rise of arid-adapted vegetation types Sage, ; Willis and McElwain, By the late Miocene 11—5 million years ago , warm, semi-arid, summer-wet climate zones were present in south-central North America, central Asia and Arabia, and northeastern Africa Willis and McElwain, Most plants exhibit alternation of generations, which is described as haplodiplontic : the haploid multicellular form known as a gametophyte is followed in the development sequence by a multicellular diploid organism, the sporophyte.
The gametophyte gives rise to the gametes, or reproductive cells, by mitosis. It can be the most obvious phase of the life cycle of the plant, as in the mosses, or it can occur in a microscopic structure, such as a pollen grain in the higher plants the collective term for the vascular plants.
The sporophyte stage is barely noticeable in lower plants the collective term for the plant groups of mosses, liverworts, and hornworts.
Towering trees are the diplontic phase in the lifecycles of plants such as sequoias and pines. The sporophyte of seedless plants is diploid and results from syngamy or the fusion of two gametes [Figure 1].
The sporophyte bears the sporangia singular, sporangium , organs that first appeared in the land plants. Inside the multicellular sporangia, the diploid sporocytes, or mother cells, produce haploid spores by meiosis, which reduces the 2 n chromosome number to 1 n. The spores are later released by the sporangia and disperse in the environment. Two different types of spores are produced in land plants, resulting in the separation of sexes at different points in the life cycle.
After germinating from a spore, the gametophyte produces both male and female gametangia , usually on the same individual. In contrast, heterosporous plants produce two morphologically different types of spores. The male spores are called microspores because of their smaller size; the comparatively larger megaspores will develop into the female gametophyte. Heterospory is observed in a few seedless vascular plants and in all seed plants.
When the haploid spore germinates, it generates a multicellular gametophyte by mitosis. The gametophyte supports the zygote formed from the fusion of gametes and the resulting young sporophyte or vegetative form, and the cycle begins anew [Figure 2] and [Figure 3].
The spores of seedless plants and the pollen of seed plants are surrounded by thick cell walls containing a tough polymer known as sporopollenin. This substance is characterized by long chains of organic molecules related to fatty acids and carotenoids, and gives most pollen its yellow color. Sporopollenin is unusually resistant to chemical and biological degradation.
Its toughness explains the existence of well-preserved fossils of pollen. Sporopollenin was once thought to be an innovation of land plants; however, the green algae Coleochaetes is now known to form spores that contain sporopollenin. Protection of the embryo is a major requirement for land plants. The vulnerable embryo must be sheltered from desiccation and other environmental hazards.
In both seedless and seed plants, the female gametophyte provides nutrition, and in seed plants, the embryo is also protected as it develops into the new generation of sporophyte. Gametangia singular, gametangium are structures on the gametophytes of seedless plants in which gametes are produced by mitosis. The male gametangium, the antheridium, releases sperm. Many seedless plants produce sperm equipped with flagella that enable them to swim in a moist environment to the archegonia, the female gametangium.
The embryo develops inside the archegonium as the sporophyte. The shoots and roots of plants increase in length through rapid cell division within a tissue called the apical meristem [Figure 4].
The apical meristem is a cap of cells at the shoot tip or root tip made of undifferentiated cells that continue to proliferate throughout the life of the plant. Meristematic cells give rise to all the specialized tissues of the plant.
Elongation of the shoots and roots allows a plant to access additional space and resources: light in the case of the shoot, and water and minerals in the case of roots. A separate meristem, called the lateral meristem, produces cells that increase the diameter of stems and tree trunks.
Apical meristems are an adaptation to allow vascular plants to grow in directions essential to their survival: upward to greater availability of sunlight, and downward into the soil to obtain water and essential minerals. As plants adapted to dry land and became independent of the constant presence of water in damp habitats, new organs and structures made their appearance.
Early land plants did not grow above a few inches off the ground, and on these low mats, they competed for light. By evolving a shoot and growing taller, individual plants captured more light.
Because air offers substantially less support than water, land plants incorporated more rigid molecules in their stems and later, tree trunks. The evolution of vascular tissue for the distribution of water and solutes was a necessary prerequisite for plants to evolve larger bodies. The vascular system contains xylem and phloem tissues. Xylem conducts water and minerals taken from the soil up to the shoot; phloem transports food derived from photosynthesis throughout the entire plant.
The root system that evolved to take up water and minerals also anchored the increasingly taller shoot in the soil. In land plants, a waxy, waterproof cover called a cuticle coats the aerial parts of the plant: leaves and stems. The cuticle also prevents intake of carbon dioxide needed for the synthesis of carbohydrates through photosynthesis.
Stomata, or pores, that open and close to regulate traffic of gases and water vapor therefore appeared in plants as they moved into drier habitats.
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