A shrub is a woody perennial plant that can be considered a short version of a tree. The plant's three main components all perform essential functions that contribute to the shrub receiving nutrients and continuing to grow. Just as some trees are poisonous so are shrubs. According to gardening website Yardener, many parts of some shrubs including the leaves, stems, flowers and roots can be poisonous if ingested.
The crown of a shrub is the leafy part of the plant that rises above ground. The crown contains branches, side-branches and leaves. The entire crown of the shrub could be considered a series of younger and older shoots of varying thickness that grow into branches, bear leaves and flower buds, according to the Natural Resources Canada website. The crown grows each year by extending new shoots. New leaves can only be produced on newly grown shoots.
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A shrub's trunk is located at the base of the plant and serves as the central anchor for the crown and root system. The trunk is the portion of plant where its many branches and root system meet; where all of the plant's vascular systems are gathered.
The root of system of a shrub sits below the topsoil level. The job of this plant part is to leach nutrients and water from the soil so that it may be delivered to shrub's crown. The root system also serves to keep the plant situated in the ground. Without a deep root system a shrub could easily blow away in high winds or easily be pulled out by plant-eating animals.