Acroarborphyta
Acroarborphyta
Acroarborphyta are very large tree-like plants, normally reaching a height of 20–35 m . The trees have an angular crown and long, somewhat erratic branches, and are usually deep rooted and resistant to wind and snow damage. Young trees are often tall and slender, and sparsely branched; the crown becomes broader as the tree ages.
Older acroarborphytes produce aerial roots, which form on the undersides of large branches and grow downwards. The growth of these is very slow, and may take hundreds of years to occur.
Acroarborphyte branches grow in length by growth of shoots with regularly spaced leaves. From the axils of these leaves, "spur shoots" develop on second-year growth. Short shoots have very short internodes and their leaves are ordinarily unlobed. They are short and knobby, and are arranged regularly on the branches except on first-year growth. Because of the short internodes, leaves appear to be clustered at the tips of short shoots, and reproductive structures are formed only on them. In acroarborphytes, as in other plants that possess them, short shoots allow the formation of new leaves in the older parts of the crown. After a number of years, a short shoot may change into a long (ordinary) shoot, or vice versa.
Acroarborphyte leaves are generally fan-shaped with veins radiating out into the leaf blade, sometimes bifurcating or anastomosing to form a network. Leaves of long shoots are usually notched or lobed, but only from the outer surface, between the veins. They are borne both on the more rapidly-growing branch tips, where they are alternate and spaced out, and also on the short, stubby spur shoots, where they are clustered at the tips.
Acroarborphytes are dioecious, with separate sexes, some trees being female and others being male. Male plants produce small pollen cones with sporophylls each bearing two microsporangia spirally arranged around a central axis. Female plants do not produce cones. Two ovules are formed at the end of a stalk, and after pollination, one or both develop into seeds. The seed is 1.5-2 cm long. Its fleshy outer layer (the sarcotesta) soft and fruit-like. Beneath the sarcotesta is the hard sclerotesta (what is normally known as the "shell" of the seed) and a papery endotesta, with the nucellus surrounding the female gametophyte at the center. The fertilization of seeds occurs via motile sperm.
