Encyclopedia of Biology

vascular cambium A secondary meristem, i.e., a
thin layer of undifferentiated plant cells that divide
indefinitely, which gives rise to secondary xylem and
phloem, leading to an increase in stem girth. Tissue
external to the vascular cambium is bark.

vascular plants The plant kingdom comprises
algae, bryophytes, seedless vascular plants, and seed
vascular plants (gymnosperms and angiosperms). Most
of the familiar flora in the United States such as trees,
shrubs, herbs, vines, grasses, ferns, and most other
land plants belong to the Tracheophyta, or vascular
plants.
These plants have systems for transporting water,
sugars, and nutrients and are differentiated into stems,
leaves, and roots. They have an elaborate system of
conducting cells, consisting of xylem, where water and
minerals are transported, and phloem, where carbohy-
drates are transported. This method of rigid internal
support enables them to stand and grow erect and dis-
tribute nutrients against gravity.
There are about 17,000 species of vascular plants
native to the United States, along with several thousand
additional native subspecies, varieties, named natural
hybrids, and about 5,000 exotic species known outside
cultivation. More than 4,850 species (about 28 per-

cent) of the native U.S. vascular plants are considered

globally rare.

Vascular plants first appear in the fossil record dur-

ing the mid-Silurian period, about 410 million years

ago. Rhyniophyta is the earliest known division of

these plants, represented by several genera. Vascularis

from Latin vasculum,meaning a vessel or duct.

Nonvascular plants, like mosses and liverworts,

have poorly developed fluid transportation systems.

vascular system Anetwork of specialized cells; the

vascular tissue that transports water and nutrients from

the roots throughout a plant’s body. In animals, it is a

specialized network of vessels—arteries, veins, and cap-

illaries—for the circulation of fluids throughout the

body tissue of an animal.

vascular tissue The collective name given to the

xylem and phloem, both tissues that carry food, water,

and minerals through the plant for the nourishment of

the cells in vascular plants.

vas deferens(sperm duct) The excretory duct or

tube that carries sperm from the epididymis, a long

coiled tube in which spermatozoa are stored, to the

338 vascular cambium

Lycopodophyta

(clubmosses)

Sphenophyta

(horsetails)

Filicinophyta

(ferns)

Bryophyta Cycadophyta

(cycads)

Ginkgophyta

(ginkgo)

Coniferophyta

(conifers)

Hepaticae

(liverworts)

Musci

(mosses)

gymnosperms

Angiospermophyta

(flowering plants)

Monocotyledonae

(e.g. grasses,

orchids, lilies)

Dicotyledonae

(e.g. oak, rose,

daisies)

* Extinct and mostly extinct groups are excluded

*Plant

The plant kingdom comprises algae, bryophytes, seedless vascular plants, and seed vascular plants (gymnosperms and angiosperms).