- ability to synthesize carbohydrates by photosynthesis

- presence of cellulose cell walls

- alternation of generations in their life cycles

- lack of mobility

Read plant diversity page 245.

Aquatic plants - The simplest of plants live surrounded by water because water satisfies many of their needs. It prevents cells from drying out, lends structural support to the plant, provides nutrients, and accommodates the dispersal of spores and the meeting of sex cells.

Complete review questions on page 251 NELSON

Read Importance of Terrestrial Plants on page 256 NELSON

What challenges were the first land plants faced with?

- air offers little support to organisms so they had to hold themselves up to the sun.

- obtaining enough water and dissolved nutrients

- transporting and distributing water, dissolved minerals, and products of photosynthesis to the entire plant.

- preventing excessive water loss by evaporation

- maintaining a proper gas exchange between the plant and the surrounding medium (air).

- fertilization and protection of the embryo

- withstanding extreme fluctuations in environmental conditions.

The evolution of higher plants of the higher plants can be best understood in terms of adaptations to these challenges.

The primary adaptations in land plants:

- roots (anchor/absorb water and nutrients), stems (support), leaves (photosynthesis), cuticle (a waxy covering over the stem and leaves assisted plants in retaining water), stomata (microscopic pores that permit the exchange of gases between the plant and the atmosphere)

- and a method of reproduction that did not depend on water

- the evolution of structures and systems for the transport of water and minerals upward from the roots to leaves and the distribution of sugar throughout the plant body.

Land plants are divided into two groups:

Bryophyta - nonvascular plants such as mosses

Tracheophytes - true land plants, such as ferns, shrubs, herbs trees, and flowering plants.

The largest and most familiar group of bryophytes are the mosses. Like other bryophytes they are restricted to swampy regions and other relatively moist environments. The simplest of land plants. The sporophyte depends on the gametophyte for water and nutrients in mosses (Figure 10.4 page 258). The moss gametophyte has organs that superficially resemble those of higher plants.

They lack the elaborate conducting tissues found in more advanced plants. The transport of water, minerals, and the products of photosynthesis in mosses depends upon simple, vertically elongated cells in the center of stems and leaves. Water moves from cell to cell by simple diffusion.

In place of complex roots found in higher plants, mosses have simple structures with a limited capacity for absorption and anchorage called rhizoids.

The sporophyte which is shorter lasting than the gametophyte and dependent on it has an elongated stalk that elevates the spore-bearing capsule called the sporangium and provides a more efficient arrangement for scattering spores. The sporangium also protects the spores as they develop.

Mosses are also essential to the formation of peat. Dried peat has been used as a fuel source in Northern European countries for years.

The tremendous absorption and antiseptic properties of moss have also facilitated its use in mopping up oil spills and as a surgical dressing in W W I.

Mosses and other bryophytes are pioneer plants. They are often the first to establish themselves on newly exposed rock or on soil surfaces where there are no plants. As mosses die off, they add to the organic matter in the soil, enriching it for larger plants. In this way, they play a role in building organic content that eventually leads to soil formation. Once pioneer species are established, they improve living conditions for other species and in so doing set the stage for their own demise.

[Complete the alternation of generations assignment.]

The division tracheophyta is comprised of the vascular plants. They include the "true" terrestrial plants such as ferns, herbs, shrubs, trees and flowering plants.

They transport water and minerals via a vascular system. This system consists mainly of xylem tissue and phloem tissue.

Xylem tissue - mainly dead cells that form microscopic tubes to convey water and minerals up from the roots.

Phloem tissue - Mainly living cells that distribute sugars throughout the plant.

The two main kinds of vascular plants are those that produce seeds and those that do not.

The most successful group of seedless plants are the ferns (Pterophyta). Ferns are extremely diverse in habitat, form, and reproductive methods. In size alone they range from 2 mm to 15 m in height. Most are shade-living plants and their numbers diminish with increasing latitude and decreasing moisture.

Ferns also play an important role in forest succession. They grow on bare, exposed rock surfaces and in open bogs and marshes in advance of the establishment of forest vegetation.

The greatest diversity of ferns is found in tropical rain forests. Many of these species grow on the trunks and branches of trees and are known as epiphytes.

Ferns have retained a primitive life cycle. The "cycle" involves two separate and relatively independent generations or growth phases that are wholly different in many respects. The conspicuous plant is the sporophyte generation, the familiar leafy plant that produces spores, often in clusters called sori on the lower surface of the leaves. Much less familiar and often overlooked is the gametophyte generation, a tiny plant resembling a moss and having a short life span. [This type of life cycle which depends on spores for dispersal existed long before the seed-plant life cycle.]

Edible ferns are called fiddleheads. Ferns are also used extensively in floral decorations.

[Complete alternation of generations in ferns assignment] Moss Fern Quiz

Algae, especially green algae play an important role in the overall global environment. They are the primary producers and the source of biological energy for nearly all food webs in aquatic environments.

Many types of algae have been used commercially in the production of foods such as pie fillers and gelatins.

What made seed plants the most successful, widely distributed, complex group of plants on earth today?

- Reproductive adaptations: seed plants have seperate male and female gametophytes.

- improved vascular system: roots, stems, and leaves.

A seed plant contains a plant embryo or a partially developed plant. Many seed plants are free living. [Many seed plants are free living, but some are saprophytic or parasitic. Most live on dry but others, such as lillies and a few trees prefer wetlands.]

Gymnosperms include the pines, spruces, junipers, firs and other cone bearing plants (coniferous). Their characteristic thin, needlelike leaves are a special adaptation to the harshness of hot, dry, summers, cold winters, and moderate rainfall. The needles are covered by a hard waxy cuticle that helps the plant to retain moisture.

Another conifer adaptation is the development of roots that extend over a wide surface area rather than penetrate deep into the soil. This feature holds the tree firm even in locations where soil is scanty and lets them grow tall (light competition).

The Pine Life Cycle - The pine has two types of cones, male and female. The male cones eventually develop into pollen grains (microspores). Pollen grains contain the male gametophyte (reduced to a very few cells). Female cones bear ovules where one spore survives and developes into a female gametophyte (megaspore). Pollen is released and carried by wind to the female cone, where it lands and germinates to produce a pollen tube that grows into the female gametophyte. The sperm cell and egg cell fuse, forming the next generation sporophyte. The sporophyte develops into an embryo encased in a seed. The seed is later released to be transported by the wind to where (hopefully) it lands and germinates. It starts as a seedling and if successful it will grow into another mature tree (sporophyte) to begin the cycle again. [Often when cutting an area, it is best to leave several of the biggest trees behind to seed the next generation.]

Xylem is a term applied to woody (lignin-impregnated) walls of certain cells of plants. Xylem cells tend to conduct water and minerals from roots to leaves. While parenchyma cells do occur within what is commonly termed the "xylem" the more identifiable cells, tracheids and vessel elements, tend to stain red with Safranin-O. Tracheids are the more primitive of the two cell types, occurring in the earliest vascular plants. Tracheids are long and tapered, with angled end-plates that connect cell to cell. Vessel elements are shorter, much wider, and lack end plates. They occur only in angiosperms, the most recently evolved large group of plants.

Phloem cells conduct food from leaves to rest of the plant. They are alive at maturity and tend to stain green (with the stain fast green). Phloem cells are usually located outside the xylem. The two most common cells in the phloem are the companion cells and sieve cells. Companion cells retain their nucleus and control the adjacent sieve cells. Dissolved food, as sucrose, flows through the sieve cells.

Plant cells are formed at meristems, and then develop into cell types which are grouped into tissues. Plants have only three tissue types: 1) Dermal; 2) Ground; and 3) Vascular. Dermal tissue covers the outer surface of herbaceous plants. Dermal tissue is composed of epidermal cells, closely packed cells that secrete a waxy cuticle that aids in the prevention of water loss. The ground tissue comprises the bulk of the primary plant body. Parenchyma, collenchyma, and sclerenchyma cells are common in the ground tissue. Vascular tissue transports food, water, hormones and minerals within the plant. Vascular tissue includes xylem, phloem, parenchyma, and cambium cells.

Plant cell types rise by mitosis from a meristem. A meristem may be defined as a region of localized mitosis. Meristems may be at the tip of the shoot or root (a type known as the apical meristem) or lateral, occurring in cylinders extending nearly the length of the plant. A cambium is a lateral meristem that produces (usually) secondary growth. Secondary growth produces both wood and cork (although from separate secondary meristems).

Secondary growth - Cells in a plant that are produced by a cambium. Increase in girth of a plant due to the action of lateral meristems such as the vascular cambium. The main cell produced in secondary growth is secondary xylem, better known as wood.

Coniferous forests besides being a habitat for numerous living organisms are an important source raw materials for a variety of commercial and industrial products. They provide about 85% of all the wood used in building and furniture construction. The pulp and paper industry uses millions of tonnes of conifers annually, mostly for the production of newspapers and other paper products.

Other conifer products include: varnishes, turpentine, disinfectants, fuels, and medicines.

Trees balance atmospheric gases by taking in CO2 and giving off O2. Unfortunately humans are upsetting this balance by cutting large sections of coniferous forests.

Conifers are important to a variety of animals for food and shelter. The bark, buds, and seeds are eaten by insects, birds, squirrels, rabbits, and many other animals.

Conifers also help comtrol flooding by absorbing rainwater through their roots and prevent topsoil from being eroded and blown away.

Gymnosperms are undoubtedly the group from which the angiosperms developed, although, as Charles Darwin noted in Origin of Species, which group "remains an abominable mystery".

Flowering plants, the angiosperms, were the last of the seed plant groups to evolve, appearing over 140 million years ago during the later part of the of the Age of Dinosaurs (the beginning of the Cretaceous, 140 million years ago). All flowering plants produce flowers. Within the female parts of the flower angiosperms produce a diploid zygote and triploid endosperm.

Fertilization is accomplished by a variety of pollinators, including wind, animals, and water. Two sperm are released into the female gametophyte: one fuses with the egg to produce the zygote, the other helps form the nutritive tissue known as

endosperm.

The angiosperms (angios = hidden) produce modified leaves grouped into flowers that in turn develop fruits and seeds. There are presently 250,000 known living species. Most angiosperms also have larger xylem cells known as vessels that improve the efficiency of their vascular systems.

Whence came the angiosperms? This was Darwin's "abominable mystery". Clearly angiosperms are descended from some group of Mesozoic-aged gymnosperm seed plant....but which one?

The classical view of flowering plant evolution suggests early angiosperms were evergreen trees that produced large Magnolia-like flowers. However, this view has recently been contradicted by the oldest fossil yet found, a 140 million year old plant found by David Dilcher and his associates.

The angiosperms underwent an adaptive radiation during the Cretaceous, and for the most part escaped the major extinctions at the end of the Cretaceous.

Flowers - Flowers are collections of reproductive and sterile tissue arranged in a tight whorled array having very short internodes. Sterile parts of flowers are the sepals and petals. When these are similar in size and shape, they are termed tepals. Reproductive parts of the flower are the stamen (male, collectively termed the androecium) and carpel (often the carpel is referred to as the pistil, the female parts collectively termed the gynoecium).

Flowers may be complete, where all parts of the flower are present and functional, or incomplete, where one or more parts of the flower are absent. Many angiosperms produce a single flower on the tip of a shoot (like the Lily). Others produce a stalk bearing numerous flowers, termed an inflorescence. Many flowers show adaptations for insect pollination, bearing numerous white or yellow petals. Others, like the grasses, oaks, and elms, are wind pollinated and have their petals reduced and often inconspicuous.

Flowering plants also exhibit the typical plant alternation of generations. The dominant phase is the sporophyte, with the gametophyte being much reduced in size and wholly dependant on the sporopohyte for nutrition. The is not a unique angiosperm condition, but occurs in all seed plants as well. What makes the angiosperms unique is their flowers and the "double fertilization" that occurs. Technically this is not double fertilization, but rather a single egg-sperm fusion (fertilization proper) plus a fusion of the second of two sperm cells with two haploid cells in the female gametophyte to produce triploid (3n) endosperm, a nutritive tissue for the developing embryo.

Developing from green algal ancestors, plants show a trend for reduction of the complexity, size, and dominance of the gametophyte generation. Plants also developed and refined the root-shoot-leaf axis with its specialized conduction cells. A third trend is the development of the seed to promote the dormancy of the embryo.

Angiosperms, flowering plants, are divided into two groups: monocots and dicots.

Monocot seeds have one "seed leaf" termed a cotyledon (in fact monocot is a shortening of monocotyledon). Dicots have two cotyledons. Both groups, however, have the same basic architecture of nodes, internodes, etc.

Note the scattered vascular bundles of the corn stem.

Note the ringed array of vascular bundles in this dicot stem (Medicago).

Monocot stems have scattered vascular bundles. Dicot stems have their vascular bundles in a ring arrangement. Monocot stems have most of their vascular bundles near the outside edge of the stem. The bundles are surrounded by large parenchyma in the cortex region. There is no pith region in monocots. Dicot stems have bundles in a ring surrounding parenchyma cells in a pith region. Between the bundles and the epidermis are smaller (as compared to the pith) parenchyma cells making up the cortex region.

Monocot roots, interestingly, have their vascular bundles arranged in a ring. Dicot roots have their xylem in the center of the root and phloem outside the xylem. A carrot is an example of a dicot root.

Monocot leaves have their leaf veins arranged parallel to each other and the long axis of the leaf (parallel vennation). An common example of this is the husk of corn or a blade of grass (both are monocots). Dicot leaves have an anastamosing network of veins arising from a mid-vein termed net vennation. Examples of dicot leaves include maples, oaks, geraniums, and dandelions.

Monocots have their flower parts in threes or multiples of three; example the tulip and lily (Lilium ). Dicots have their flower parts in fours (or multiples) or fives (or multiples). Examples of some common dicot flowers include the geranium, snapdragon, and citrus.

Secondary growth is produced by a cambium. It occurs in rows or ranks of cork, secondary xylem or secondary phloem cells. Cork cells (produced by a cork cambium) are technically part of the epidermis, and contribute to the bark of woody stems.

Dicot secondary growth occurs by growth of vascular cambium, to complete a full vascular cylinder around the plant. Secondary xylem is produced to the inside of the vascular cambium, secondary phloem to the outside. The living parts of the

woody plant are next to the vascular cambium.

At the end of each growing season, the vascular cambium stops growing, forming a growth ring.

Monocots usually don't have secondary growth. Some, such as bamboo and palm trees, have secondary growth. Monocot secondary growth differs from dicot secondary growth in that new bundles are formed at the edge of the stem. These new bundles are close together, providing support for the stem.