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The transition of plants from aquatic to terrestrial habitats occurred approximately 400 million years ago in the Silurian geological period. The first tracheophytes, which were very small plants with conducting tissues, broke the barrier between water and air. This represented a transition from rather constant environmental conditions to unpredictable and changeable ones. It is therefore not surprising that the stems of the first terrestrial plants already had almost all of the characteristics associated with recently living vascular plants: a protective peripheral layer, an epidermis, a parenchymatic cortex and a central cylinder, consisting of tracheids which conduct water and sieve elements which transport assimilates.
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