Botany for Gardeners (Science for Gardeners)

by Brian Capon

During the second phase of photosynthesis, carbon dioxide fixation, carbon dioxide from the atmosphere unites with the sugar ribulose diphosphate. The product is split into two equal parts, hydrogen from the light reaction is added, and the resulting molecules of PGAL (phosphoglyceraldehyde) are then used as small building blocks for the construction of more elaborate molecular forms, using the stored energy from the light reaction. Although carbon dioxide fixation is slower than the light reaction, millions of PGAL molecules are synthesized within minutes after light enters a leaf’s mesophyll tissues.

Thousands of glucose molecules are united into long chains, forming the huge molecules of starch and cellulose.

Starch is the principal food stored in plant cells. It can be broken into component glucose units when they are needed as an energy source in respiration or for conversion into other plant products by the specialized activities of enzymes.

One of nature’s most remarkable and little-known group of plants is the diatoms, which are unicellular algae of vast numbers floating in marine and freshwater habitats. There are approximately 100,000 species of diatoms living today. The diatoms’ cells are encased in unique cell walls made of silica that have an astounding diversity of ornate forms and consist of two halves that typically overlap like the two halves of a pill box.

Plants produce two types of reproductive cells. The first is the fine, dust-like particles issuing from brown spots on the underside of fern leaves or from inside mushroom caps, for example; these are called spores.

The second reproductive cell is formed during sexual reproduction, a more complicated system for producing offspring because it requires a population to be divided into male and female members or, at least, the development of distinct male and female structures on individual plants.

The sex cells, or gametes (Greek: gamein, “to marry”), are incapable of growing directly into new plants, as do spores. In sexual reproduction, a sperm (male gamete) unites with an egg (female gamete) to form a single cell, the zygote (Greek: zygotos, “yoked, united”). Starting with this one cell, repeated divisions lead to a multicellular organism.

In many terrestrial habitats water is not plentiful at the time of reproduction. Thus, to ensure the success of gamete transfer in gymnosperms and angiosperms, the sperm move through liquid-filled pollen tubes that first make contact with the awaiting egg. Reproductive processes, vital to the future of each species, have evolved to a state of extreme precision.