Page:An introduction to physiological and systematical botany (1st edition).djvu/55

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Under the Cellular Integument we find the Bark, consisting of but one layer in plants or branches only one year old, and often not distinguishable from the wood. In the older branches and trunks of trees, it consists of as many layers as they are years old, the innermost being called the liber; and it is in this layer only that the essential vital functions are carried on for the time being, after which it is pushed outwards with the Cellular Integument, and becomes like that a lifeless crust. These older layers, however, are for some time reservoirs of the peculiar secreted juices of the plant, which perhaps they may help to perfect.

In some roots the bark, though only of annual duration, is very thick; as in the