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Chapter: Introduction to Botany: The Origin of Flowering

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Upgraded Gymnosperms

Flowering plants (angiosperms, Angiospermae) are sometimes referred to as “Spermatophyta 2.0.”, or “upgraded gymnosperms”.

Upgraded Gymnosperms

 

Flowering plants (angiosperms, Angiospermae) are sometimes referred to as “Spermatophyta 2.0.”, or “upgraded gymnosperms”. In fact, there is no single character which unequivocally differs flowering plants from other seed plants. Only several characteristics combined together will distinguish angiosperms. Flowering plants have their ovules inside an additional cover: pistil which cor-responds with megasporophyll (sporangium-bearing leaf); later, the pistil de-velops into the fruit. These plants have an almost complete reduction of game-tophytes: three or even two cell of the pollen (male gametophyte) and seven (sometimes even four) cells in embryo sac (female gametophyte), there are no archegonia or anteridia. Like gnetophytes, they have double fertilization. The sperms (spermatia) come through the pollen tube (like in conifers and gneto-phytes). One sperm fertilizes the egg cell, and the other sperm fertilizes the biggest cell of embryo sac (Fig. 8.1).


 

While the first fertilization results in a “normal” diploid zygote which grows into embryo, the second fertilization ignites the process of feeding tissue develop-ment. This feeding tissue is endosperm2, frequently triploid (3n) since it orig-inates from the sperm and cell with two nuclei and sperm, or diploid (2n), if thebiggest cell of embryo sac (central cell) had one nucleus only (Fig. 8.2).

 

Double fertilization may be explained in several ways: (1) the second fertil-ization results in second, “altruistic” embryo which sacrifices itself to feed the “brother”; (2) second fertilization is only a signal which initiates the develop-ment of endosperm and it does not really matter which genotype it has; and/or (3) to make a functional nutrition tissue, angiosperms need a polyploid genome whereas its origin is not so important.

 

One way or another, flowering plants abandoned the pre-fertilization development of the nutrition tissue, and changed endosperm1 to endosperm2.

 

In the Mesozoic era, gymnosperms were the dominating plants of the tree story. However, in the uderstorey, herbaceous spore plants did not surrender to seed plants and were still dominating. Amazingly, there were almost no herbaceous gymnosperms! The explanation is that gymnosperms, being quite advanced in general, had a slow and ineffective life cycle. Ineffectiveness was in part due to the absence of sophisticated cross-pollination like insect pollination (which re-quires edible parts like nectar or excess pollen). In addition, since gymnosperm fertilization occurs after gametophyte development, there is frequently a waste of resources: if fertilization does not occur, then all nutrition tissue (endosperm1) will be lost; such empty seeds are unfortunately not rare among gymnosperms.

 

If gymnosperms were to increase the speed of life cycle, make more sexual struc-tures, grow rapidly, improve vegetative reproduction, make better pollination and seed dispersal, they could win the competition with ferns in the under-story.

 

This is exactly what happened with flowering plant ancestors. Flowering plants grow fast and restore missing (eaten) parts with high speed, they parcel-late (clone from body parts) easily, they have small and numerous floral units(flowers) which are frequently bisexual but protected from self-pollination and adapted to insect pollination, they guard ovules with pistil wall, their pollen tube grows in hours (not days and weeks), they use fruits to distribute seeds.

 

Fertilization of angiosperms involves the signaling event: when second sperm fertilizes central cell, it “rings a bell” saying that the first fertilization is now completed. Endosperm (endosperm2 in that case) will start to develop only after the fertilization, and resources will not be wasted. This agile life cycle is the main achievement of angiosperms.

 

There is a growing evidence that these ancestors were paleoherbs, herbaceous plants (and maybe, even water plants like one of the most primitive angiosperms, fossil Archaefructus, or quite basal extant Ceratophyllum). Right after they won a competition with herbaceous spore plants, they started to conquer the tree storey again, and now, angiosperms dominate the Earth. There are more than 250,000 species of them which is more than any other group of living beings ex-cept insects. There are about 300 families and around 40 different orders. The only places that angiosperms do not grow are the open ocean and the central Antarctic.

 

The life cycle of angiosperm (Fig. 8.3) begins much like that of other seed plants; however, when it reaches the point of fertilization, it changes. The male game-tophytes, pollen grains, produce pollen tubes which rapidly grow to the ovule and deeper, to the embryo sac. The embryo sac typically has seven cells and eight nuclei (two nuclei in the central cell). The first sperm fertilizes the egg and produces the zygote whereas the second sperm fertilizes the central cell and produces the mother cell of the endosperm2:

 

1.              1st sperm cell (1st spermatium, n) + egg cell (n) - >  zygote (2n)

 

2.              2nd sperm cell (2nd spermatium, n) + central cell (2n or sometimes n) - > mother cell of endosperm2 (3n or sometimes 2n)

 

At the end of life cycle, the flowering plant develops the fruit (Fig 8.4). Each part of the fruit is of different origin: fruit skin and wall are from mother plant pistil, seed coat is from mother plan ovule, endosperm2 is a result of second fertilization, and embryo is a daughter plant resulting from the first fertiliza-tion. What is interesting, is that the embryo of angiosperms is still parasitic: it lives on endosperm which originates from (fertilized, ignited) cell of female gametophyte—in essence, still similar to mosses.


 

Angiosperms have four subclasses: Magnoliidae being the most primitive with flowers of numerous free parts (like water lily, Nymphaea); liliids (Liliidae, mono-cots) are grasses, palms, true lilies and many others with trimerous flowers; rosids (Rosidae) with pentamerous or tetramerous flowers and free petals; and most advanced are asterids (Asteridae) bearing flowers with fused petals and reduced number of carpels (Fig. 8.5).

 


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