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Ferns and whisk ferns belong to the division Pterophyta. A third group of plants in the Pterophyta, the horsetails, is sometimes classified separately from ferns. Horsetails have a single genus, Equisetum. They are the survivors of a large group of plants, known as Arthrophyta, which produced large trees and entire swamp forests in the Carboniferous. The plants are usually found in damp environments and marshes (Figure 14.14).
![media/image15.png](http://www.opentextbooks.org.hk/system/files/resource/34/34471/35013/media/image15.png)
The stem of a horsetail is characterized by the presence of joints, or nodes: hence the name Arthrophyta, which means “jointed plant”. Leaves and branches come out as whorls from the evenly spaced rings. The needle-shaped leaves do not contribute greatly to photosynthesis, the majority of which takes place in the green stem (Figure 14.15).
![media/image16.png](http://www.opentextbooks.org.hk/system/files/resource/34/34471/35013/media/image16.png)
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