“One defining feature of cacti is having clusters of spines. Numerous plants have spines of course, but in cacti, spines occur in clusters in the axil of leaves, even though the leaves are usually microscopic. Most cactus morphologists have concluded that cactus spines are either modified leaves or modified bud scales (the difference is inconsequential because bud scales themselves are modified leaves). The leaf-nature of spines is certainly understandable from the point of view of location: spine primordia look just like leaf primordia and are produced at a location where we would expect leaf primordia – at the base of the axillary bud’s shoot apical meristem.
“Evolution appears to have been more complex than would be expected: mature cactus spines do not contain any of the cells or tissues characteristic of leaves, and conversely leaves lack all features characteristic of spines. The two organs have little in common other than developing from leaf primordia. Spines consist of just a core of fibers surrounded by sclereid-like epidermis cells. They have no stomata, no guard cells, no mesophyll parenchyma, no xylem, no phloem. When mature, all cells in a spine are dead, and even when the spine is still growing it has living cells only at its base. Cactus leaves on the other hand … have parenchymatous epidermis cells, guard cells, spongy mesophyll, chlorenchyma, xylem and phloem. So the evolutionary conversion of cactus leaves into spines did not involve a mere reduction of the lamina and then further reduction of midrib and petiole, it instead involved the suppression of all leaf-cell type genes and activation of genes that control formation of fibers, the deposition and lignification of secondary walls, and then programmed cell death. These fiber morphogenesis genes are not activated in any cactus leaf (none at all has fibers), but they are activated of course in the development of wood. It would appear that after an axillary bud apical meristem initiates spine primordia, most leaf genes remain suppressed and instead wood fiber genes are activated. This does not involve all wood genes because vessels are never produced in the spines, just wood fibers. This would be a type of homeotic evolution.”
Mauseth, J. D. 1982. Development and ultrastructure of extrafloral nectaries in Ancistrocactus scheeri (Cactaceae). Botanical Gazette 143: 273 – 277.
Ground Tissues consist primarily of thin-walled parenchyma cells. Other ground tissue is composed of collenchyma cells and sclerenchyma cells
most common type of plant cell
large vacuoles, thin walls, and are initially(briefly) more or less spherical
living protoplasts push up against each other shortly after being produced and produce other shapes, ending up with 11 to 17 sides.
function in storage of food and water, photosynthesis, and secretion
most abundant cells of primary tissue and may also occur in a much lesser extent insecondary tissues.
most only have primary walls(walls laid down while the cells are still maturing)
less specialized than other plant cells, although many variations occur with special functions ( nectar and resin secretion or storage of latex, proteins, and metabolic waste).
functional nuclei and are capable of dividing
remain alive after maturation; (100 years old in cacti)
some contain chloroplasts (especially in leaves and other outer plant parts)- photosynthetic parenchyma tissue called chlorenchyma
have living protoplasts and may live for many years
usually a little longer than wide
have walls that vary in thickness
provide support for plant organs, allowing them to bend without breaking(flexible)
often form strands or continuous cylinders beneath the epidermis of stems or stalks and along the veins in leaves.
provide much support for stems in primary plant body.
example: celery strings.
tough, thick walls.
usually lack living protoplasts at maturity (unlike collenchyma and parenchyma)
secondary cell walls impregnated with lignin
lignin -highly branched polymer that makes cell walls more rigid. cell walls containing this are considered lignified. Common in walls of plant cells that have a structural or mechanical function. Could be in primary walls as well.
two types: fibers and sclereids; both serve to strengthen the tissues in which they occur
fibers - long, slender cells that are usually grouped together in strands.
sclereids- variable in shape but often branched.May occur singly or in groups; they are not elongated, but may have many different forms (one is a star). Found in hard seed coats. Cause gritty texture of pair.
BIOL 1105/1106 Virginia Polytechnic Institute and State University