Why does a praying mantis have one ear, an octopus have three hearts and a leech have 32 brains?
These complex patterns of gene expression appear to be the preserve of eukaryotes—creatures built of cells that have their primary genomes wrapped up in a complex compartment called a nucleus. Sponges, descended from some of the earliest multicellular creatures, have a handful of cell types. Plants have dozens. Complex animals have hundreds
What is more, animals tend to have a standard complement of each type of organ. A healthy oak tree may have 100,000 leaves or 300,000 leaves. There is no proper number of blossoms for a cherry.
Photosynthesis also needs water. This is drawn up from the roots through tubes called xylem and evaporates within a leaf, moistening the air that the mesophyll cells make use of. Other tubes, called phloem, carry away the sugars created through photosynthesis. Air moves in and out through openings in the leaf’s cuticle, called stomata. If water is plentiful the stomata dilate, allowing lots of carbon dioxide in.
Most of these organs are physically coherent. The co-ordination of their cells is achieved by structural linkage between them. In some cases you can say where they begin and end well enough that it is possible to cut one out of one person and put it into another. Some, though, are distributed—most notably the immune system, which has outposts throughout the body and a range of single-celled components coursing through the blood.
In the main, the various types of neuron have nuclei which are, together with their immediately surrounding cytoplasm, located in the brain or the spinal cord. But they also have protuberances called axons, which reach out from them, in some cases into the rest of the body, and so may stretch for a metre or more. These axons are wrapped in sheaths of a fatty material called myelin, secreted by specialised glial cells. Loss of this myelin has bad effects, such as multiple sclerosis.
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