Plant cells have elaborate structures (nucleus, chloroplasts, mitochondria, vacuoles and so on) and a complex web of metabolic pathways. In many plant cells, cytoplasmic fluxes are visible in the light microscope, so there are internal transport processes. Structure, metabolism and transport depend on one another as they do in animal cells. In short, plant cells exhibit internal states as defined in chapter 6.
Every plant has its own distinctive genome. Different combinations of genes are expressed in different cells. Changes in internal state, such as increased photosynthesis, increase the transcription of particular genes; the expected dialogue between internal state and gene expression is apparent.
Plant cells respond to external stimuli. Increased sunlight opens pores in the leaves, facilitating exchanges of carbon dioxide and oxygen, compatible with increased photosynthesis. Specific signals promote the growth and differentiation of structures involved in reproduction. Thus, external stimuli can bring about changes in both internal state and gene expression pattern. As in animal cells, the processing of stimulus information requires the manufacture of signalling pathway components (gene products) and their correct location and chemical condition (aspects of internal state).
The three-way dialogue model therefore applies to plants as well as to animals. Our characterisation of "livingness" applies just as well to plant cells as it does to animal cells. However, there are obvious differences between animals and plants. Ecologically, plants are "primary producers"; they manufacture living material out of inorganic substances and external energy. They take water, nitrate and carbon dioxide as nutrients and use a non-living energy source (sunlight). Animals, in contrast, are "consumers"; they obtain their energy and materials from other organisms. They eat either plants or animals that have eaten plants. This basic difference is represented in body design and the overall speeds of life processes: plants are generally less compact than animals and their life processes are slower.
Many animal cells are attached to a framework known as the extracellular matrix, much as sweet peas are attached to a trellis. The extracellular matrix is secreted by certain types of cells and provides support for others, organising their relative dispositions in space and conferring shape on tissues and on the whole body. Plant cells have no extracellular matrix, but unlike animal cells they have tough cell walls. These cell walls can be modified and thickened, lending rigidity to tissues and helping to define shape. The most and familiar example of wall thickening and transformation is the formation of wood. The cells elongate and produce very thick rigid walls with tiny pores opening from cell to cell, then the contents are lost. Wood is a bundle of long narrow thick-walled tubes made of dead elongated cells joined end-to-end by pores. The tubes serve to transport fluids (sap); the thick walls give strength and rigidity18. Wood and other rigid tissues fulfil some of the roles that the extracellular matrix fulfils in animals.
Plant reproduction requires the production of spores or seeds. Spores and seeds contain the DNA needed to make a new organism more or less identical to the parent(s)19. Seeds (though not spores) usually contain reserve fuel for the plant's growth until it can take care of its own nutrition.
Spores are interesting objects. Encased in tough, resistant coats for protection, they consist of one or a few undifferentiated cells that do nothing. These cells exchange no materials or energy with their surroundings. They do not metabolise. They transcribe no genes, make no proteins. They do not respond to their surroundings unless and until the environment is suitable for a new individual plant to grow. Only then does the spore lose its tough coat and germinate; that is, the cells wake up and become active, and growth and development begin.
Can spores be described as "living"? According to our characterisation they cannot. The cells have internal structures but no metabolism or internal transport processes, so they have no internal states able to change from moment to moment. Moreover, they express no genes and respond to no external stimuli until germination begins, i.e. until the spore ceases to be a spore. If there is no internal state, no gene expression and no stimulus response, then spores are not alive. But they are potentially alive, the potential residing in their cellular organisation and a full complement of genes that constitutes a "recipe for making an organism". Spores and seeds are in states of "suspended animation" that can last almost indefinitely. There are well-authenticated instances of spores germinating after thousands of years. Samuel Pepys records the appearance of mustard fields on sites cleared by the Great Fire of London, sites where mustard had not grown since the Roman occupation of Britain.
18 The "grain" and the mechanical strength of timber depend on the size of the tubes and the thickness of the walls.
19 Some spores (and all seeds) are the results of sexual reproduction, in which case there are two parents. Others are produced asexually and are genetically identical to their one parent.
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