PLANTS AND THEIR STRUCTURE II
Angiosperms,
flowering plants, are divided into two groups: monocots and dicots.
Features of
monocot and dicot plants. Images from Purves et al., Life: The Science of
Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH
Freeman (www.whfreeman.com), used with permission.
Monocot seeds
have one "seed leaf" termed a cotyledon (in fact monocot is a
shortening of monocotyledon). Dicots have two cotyledons. Both groups, however,
have the same basic architecture of nodes, internodes, etc.
Comparison of
monocot (left, oat) and dicot (right, bean) gross anatomy. Image from Purves et
al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com)
and WH Freeman (www.whfreeman.com), used with permission.
The above
images is from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Stem/Zea_cross_section/Stem_composite.
Note the scattered vascular bundles of the corn stem.
The above image
is from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Stem/Medicago_cross_section/Labeled. Note the ringed array of vascular bundles in this dicot stem (Medicago).
Monocot stems
have scattered vascular bundles. Dicot stems have their vascular bundles in a
ring arrangement. Monocot stems have most of their vascular bundles near the
outside edge of the stem. The bundles are surrounded by large parenchyma in the
cortex region. There is no pith region in monocots. Dicot stems have
bundles in a ring surrounding parenchyma cells in a pith region. Between the
bundles and the epidermis are smaller (as compared to the pith) parenchyma
cells making up the cortex region. Monocot roots, interestingly, have their
vascular bundles arranged in a ring. Dicot roots have their xylem in the center
of the root and phloem outside the xylem. A carrot is an example of a dicot
root.
Diagram
illustrating the tissue layers and their organization within monocot and dicot
roots. Image from Purves et al., Life: The Science of Biology, 4th
Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com),
used with permission.
Cross-section
of a root of corn. Note the ringed array of vascular bundles in this Zea
(monocot) root cross section. The above image is cropped and reduced from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Root/Monocot_Roots/Zea_Monocot_Root/Zea_xs.
Monocot leaves
have their leaf veins arranged parallel to each other and the long axis
of the leaf (parallel vennation). An common example of this is the husk of corn
or a blade of grass (both are monocots). Dicot leaves have an anastamosing
network of veins arising from a mid-vein termed net vennation. Examples of
dicot leaves include maples, oaks, geraniums, and dandelions.
Monocots have
their flower parts in threes or multiples of three; example the tulip and lily
(Lilium ). Dicots have their flower parts in fours (or multiples) or
fives (or multiples). Examples of some common dicot flowers include the
geranium, snapdragon, and citrus.
Secondary
growth is produced by
a cambium. It occurs in rows or ranks of cork, secondary xylem or secondary phloem
cells. Cork cells (produced by a cork cambium) are technically part of
the epidermis, and contribute to the bark of woody stems.
Dicot secondary
growth occurs by growth of vascular cambium, to complete a full vascular
cylinder around the plant. Secondary xylem is produced to the inside of the
vascular cambium, secondary phloem to the outside. The living parts of the
woody plant are next to the vascular cambium.
Cross-section
of a young stem of basswood. Note the primary growth in cross section of a
young Tilia (basswood) stem. The above image is cropped from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Woody_Stems/Tilia_Stem_-_cross_sections/Primary_Growth/Whole_Cross_Section.
Three
cross-sections of older basswood twigs. Note the annual growth rings and the
complete vascular cylinder producing secondary xylem to the inside and
secondary phloem to the outside. The above image is from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Woody_Stems/Tilia_Stem_-_cross_sections/Secondary_Growth/1%2C_2%2C_and_3-year_old_stems.
At the end of
each growing season, the vascular cambium stops growing, forming a growth
ring.
Closeup of a
cross-section of basswood growth ring. Note the growth ring, which is formed by
very small cells followed by large cells with the commencement of growth in the
next growing season. The above image is cropped from gopher://wiscinfo.wisc.edu:2070/I9/.image/.bot/.130/Woody_Stems/Tilia_Stem_-_cross_sections/Secondary_Growth/Secondary_Xylem_-_growth_ring.
Details of the
stem of basswood. The above image is from http://www.mancol.edu/science/biology/plants_new/anatomy/grndd.html.
Monocots
usually don't have secondary growth. Some, such as bamboo and palm trees, have
secondary growth. Monocot secondary growth differs from dicot secondary growth
in that new bundles are formed at the edge of the stem. These new bundles are
close together, providing support for the stem.
The leaf consists of the
(generally) flat blade, one or more leaf veins, a petiole, and usually
an axillary bud. The petiole can be long (as in celery and bok-choy) or
short (as in cabbage and lettuce). Leaves may be simple or compound:
simple leaves have a single subdivision or leaflet, compound leaves have more
than one leaflet. Leaves attach to stems at nodes (internodes are
the spaces between nodes). Leaf phyllotaxy is the pattern exhibited (spiral,
opposite, alternate, whorled) of leaf attachment to a stem.
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