Annulus (botany)
An annulus in botany is a single row of specialized cells on the outer rim of a fern sporangium that serves in spore dispersal. It consists typically of a ring or belt of dead water-filled cells with differentially thickened cell walls that stretches about two-thirds around each sporangium in leptosporangiate ferns. The thinner walls on the outside allow water to evaporate quickly under dry conditions. This dehiscence causes the cells to shrink and a contraction and straightening of the annulus ring, eventually rupturing the sporangial wall by ripping apart thin-walled lip cells on the opposite side of the sporangium. As more water evaporates, air bubbles form in the cells causing the contracted annulus to snap forward again, thus dislodging and launching the spores away from the plant. The type and position of the annulus is variable (e.g. patch, apical, oblique, or vertical) and can be used to distinguish major groups of leptosporangiate ferns.
Footnotes
Annulus
Annulus (or anulus) or annular may refer to:
Human anatomy
Other
Annulus (mathematics)
In mathematics, an annulus (the Latin word for "little ring", with plural annuli) is a ring-shaped object, especially a region bounded by two concentric circles. The adjectival form is annular (as in annular eclipse).
The open annulus is topologically equivalent to both the open cylinder S1× (0,1) and the punctured plane. Informally, it has the shape of a hardware washer.
The area of an annulus is the difference in the areas of the larger circle of radius R and the smaller one of radius r:
The area of an annulus can be obtained from the length of the longest interval that can lie completely inside the annulus, 2*d in the accompanying diagram. This can be proven by the Pythagorean theorem; the interval of greatest length that can lie completely inside the annulus will be tangent to the smaller circle and form a right angle with its radius at that point. Therefore, d and r are the sides of a right angled triangle with hypotenuse R and the area is given by:
The area can also be obtained via calculus by dividing the annulus up into an infinite number of annuli of infinitesimal width dρ and area 2πρ dρ and then integrating from ρ = r to ρ = R:
Annulus (oil well)
The annulus of an oil well is any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept. The presence of an annulus gives the ability to circulate fluid in the well, provided that excess drill cuttings have not accumulated in the annulus, preventing fluid movement and possibly sticking the pipe in the borehole.
For a new well in the process of being drilled, this would be the void between the drill string and the formation being drilled. An easy way to visualise this would be to stand a straw (purple in diagram) straight up in the center of a glass of water. All of the water in between the straw and the sides of the glass would be the annulus (yellow area in diagram), with the straw itself representing the drill string and the sides of the glass representing the formation. While drilling, drilling fluid is pumped down the inside of the drill string and pushes the drill cuttings up the annulus to the surface, where the cuttings are removed from the drilling fluid (drilling mud) by the shale shakers.
