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Understanding Trees: Angiosperms and Gymnosperms

Conifers were once the dominant tree on earth.

Of all the complex plants we recognise, most are seed-bearing plants (spermatophytes). This includes all flowering plants and evergreen trees, but excludes ferns, horsetails and other clades (group of organisms that share a single ancestor) that reproduce through spores.

Spermatophytes can be divided into gymnosperms and Angiosperms. Both require the transfer of pollen to facilitate fertilisation, but only angiosperms rely on novel methods of pollination. Gymnosperms predominantly rely on the wind.

Angiosperms are unique in that they have flowers. Most species have flowers with both male and female organs. Stamens are the pollen-producing organs and are known collectively as the androecium. Carpels are the female organ(s) and are known collectively as the gynoecium. Carpels are composed of an upper, fertile part and lower, sterile part known as the style and ovary respectively. At the top of the style is the stigma on which pollen grains land, prior to the fertilisation of the immature seeds or ovules within the ovary. This explains angiosperms’ Latin-Greek roots angi-sperma “enclosed-seeds”.

flower parts

In the above image, the plant has one carpel, but some flowers possess multiple carpels, which can lead to different or separate ovaries. Below you can see blue passion flower, with five stamens and three carpels, leading to a single ovary.

blue_passion_flower

When compatible pollen lands on a stigma, it germinates, forming a pollen tube. The tube grows down the tissue of the style and deposits sperm for the fertilisation of the ovules. In the ovary, the pollen fuses with the ovule, producing a zygote, which develops into an embryo. Eventually, the ovules form seeds and the ovary Fruit.

Poor fertilisation (a lack of fertilised ovules) leads to abnormal development and asymmetrical fruits. Fruit size is positively correlated with seed number in such species as strawberry, kiwifruit and apple. So, as a rule, the more seeds a fruit contains the bigger it is.

The number of carpels determine a fruit’s anatomy with single carpel flowers producing a fruit with one or two (1-2) seeds such as peaches, cherries and other species in the Prunus genus. Multiple carpel flowers produce fruit with more than two (2+) seeds such as apples, pears, quinces and so on.

The positioning of the carpels vis-a-vis other floral organs also determines a fruit’s anatomy. Flowers can be divided into 3 categories: hypogynous, perigynous and epigynous, which produce persimmon type, apple type and peach type fruit respectively.

An interesting fact about epigynous flowers is their ability to produce parthenocarpic or seedless fruit, owing to the ovary’s close attachment to surrounding tissue. As a result, the Williams’ Bon Chrétien pear can sometimes produce fruit in the absence of pollination. (Despite this, the fruit will likely be smaller due to the absence of growth-hormones usually released when a flower is pollinated.)

Not all angiosperms have perfect flowers – flowers with both male and female organs. Some have separate male (staminate) and female (pistillate) flowers (known as imperfect flowers), sometimes on the same tree, sometimes on separate trees. Some bear both perfect flowers and staminate and pistillate flowers. Trees with both staminate and pistillate organs are monoecious and trees with either staminate and pistillate flowers are dioecious.

These different flowers can be explained in the fact that they have evolved to encourage pollination by wind or animals. Male wind-pollinated flowers have lightweight pollen grains, exposed anthers. Female wind-pollinated have outstretched stigmas with large surface areas. Both forgo showy petals.

The male and female flowers of a walnut. Picture credit: Dalgial licensed under CC BY-SA 3.0.

Animal-pollinated flowers are necessarily perfect with male and female organs on a single flower. They are set up to attract, reward and facilitate contact with pollinators with large, colourful petals, large, nutritious pollen grains, nectaries (easily available nectar) and long stigmas.

Nectaries of a camellia. Picture credit: Daiju Azuma licensed under  CC BY-SA 3.0.

Gymnosperms produce unenclosed seeds, hence the Greek gymnospermos meaning naked-seeds. They lack the fancy features of angiosperms such as calyx, corolla, stamens and carpels as they rely on wind-pollination. Most species consist of pollen-producing cones (staminate strobili) and seed-producing cones (ovulate strobili).

At one point in time all trees on earth were gymnosperms. Angiosperms emerged in the Triassic period, and by the Early Cretaceous period had developed flowers. Flowers conveyed a powerful evolutionary advantage as by the Late Cretaceous period they had become dominant.

As most plants need to be fertilised to produce offspring, the offspring inevitably contains genetic information from both the male and female reproductive cells (gametes). This means the offspring is different than the parent plant, even in the case of asexual reproduction. (This is due to genetic recombination and Mendel’s law of segregation. In essence, the parent’s genome is rearranged.)

By using animals as a means to reproduce, angiosperms were able create offspring with genetic information from further afield. This increased genetic variability, creating offspring better adapted to different environments. Self-sterility played an important role in this by forcing certain varieties to seek radically different genetic information.

(With self-sterile plants, genetically similar pollen tubes are destroyed, preventing fertilisation of the ovules. This occurs as the carpel produces an enzyme that penetrates the pollen tube. The enzyme is either destroyed by the pollen’s protein or left alone. If left alone, the enzyme inhibits the pollen’s mechanism of building protein and it stops growing, preventing inbreeding.)

Flowers produced a huge disparity in species between the clades with around 300,000 angiosperms existing today, but only around 1,000 gymnosperms. The remaining major divisions of gymnosperms include pinophyta (conifers), cycadophyta (cycads), ginkgophyta and gnetophyta.

The most famous, conifers, still occupy large areas of the world and are sought for their fast-growing timber. They are suited to cold regions owing to their needle-shape leaves that both reduce evaporation and ensure snow slides easily off, preventing branch breakages. Cycads are unique among gymnosperms in that they evolved to be pollinated by beetles. Ginkgophyta only contains a single living species, the Ginkgo or Maidenhair tree.

Angiosperms contain nearly all other complex plants and are of vital importance to humans, producing all fruit with the exception of pine and ginkgo nuts. Their showy flowers make them popular with gardeners, although evergreen conifers are often used by landscapers to provide screening.

Jorge at PrimroseJorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!

His ideal afternoon would involve a long walk, before settling down for scones.

Jorge is a journeyman gardener with experience in growing crops.

See all of Jorge’s posts.

The post Understanding Trees: Angiosperms and Gymnosperms appeared first on Primrose Blog.



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