De Zoeker ("The Seeker") is the name of an oil windmill, located in the Zaanse Schans, Zaanstad.
 Its purpose is to press seeds such as linseed and rapeseed into vegetable oil.
 It is the only oil mill still in operation, and is one of five remaining oil mills in the area.
Hesperolinon adenophyllum is a rare species of flowering plant in the flax family known by the common names glandular dwarf flax and glandular western flax.
 It is endemic to California, where most known occurrences have been recorded in Lake and Mendocino Counties.
 It is generally found in chaparral ecosystems on serpentine soils.
 This is an annual herb growing erect to heights between 10 and 50 centimeters.
 Its narrow, lance-shaped leaves are lined with rows of teeth with large knobby glandular points.
 The flower has small sepals speckled with glandular hairs and five thin yellow petals.
 The stamens protrude from the face of the flower and hold large yellow anthers.
Flax (also known as common flax or linseed), "Linum usitatissimum", is a member of the genus "Linum" in the family Linaceae.
 It is a food and fiber crop cultivated in cooler regions of the world.
 The textiles made from flax are known in the Western countries as linen, and traditionally used for bed sheets, underclothes, and table linen.
 The oil is known as linseed oil.
 In addition to referring to the plant itself, the word "flax" may refer to the unspun fibers of the flax plant.
 The plant species is known only as a cultivated plant, and appears to have been domesticated just once from the wild species "Linum bienne", called pale flax.
Linum grandiflorum (syn.
 L. coccineum) is a species of flax known by several common names, including flowering flax, red flax, scarlet flax, and crimson flax.
 It is native to Algeria, but it is known elsewhere in Northern Africa, Southern Europe and in several locations in North America as an introduced species.
 It is an annual herb producing an erect, branching stem lined with waxy, lance-shaped leaves 1 to 2 centimeters long.
 The inflorescence bears flowers on pedicels several centimeters long.
 The flower has 5 red petals each up to 3 centimeters long and stamens tipped with anthers bearing light blue pollen.
 It can on occasion be found as a casual well outside its normal established range; records from the British Isles, for example, are reasonably frequent (as per the latest BSBI atlas) but, grown as an annual, it rarely persists for more than one season.
Linum marginale, known by the common name of 'native flax' or 'Australian flax', is a short lived perennial flowering herb, native to Australia.
 A slender, wispy, upright plant, growing to around 1 metre high, Native Flax is often overlooked when not in flower.
 It should not be confused with species of Wahlenbergia, which occurs in the same area and can appear similarly.
 Like most species of Linum, Linum marginale can be used to produce useful fibre, but is not grown on a commercial level for this purpose.
Hesperolinon serpentinum (now "Hesperolinon sharsmithae") is a rare species of flowering plant in the flax family known by the common name Napa dwarf flax, or Napa western flax.
 "Hesperolinon serpentinum" was not validly published by McCarten although it appears in the Jepson Manual (Hickman, 1993).
 The species was validly published as "Hesperolinon sharsmithae" in 2006 by O'Donnell (Madroño 53:404–408).
 The species is endemic to northern California, where it is known from fewer than twenty occurrences in four counties surrounding the San Francisco Bay Area.
 It is a member of chaparral plant communities, generally on serpentine soils.
 It produces a thin, erect stem up to 30 centimeters tall, with sparse flat, narrow leaves.
 The inflorescence holds several light yellow flowers with petals only 2 or 3 millimeters long.
 The plant is threatened mainly by development of its habitat.
New Zealand flax describes the common New Zealand perennial plants "Phormium tenax" and "Phormium colensoi", known by the Māori names "harakeke" and "wharariki" respectively.
 Although given the common name 'flax' they are quite distinct from the Northern Hemisphere plant known as flax ("Linum usitatissimum")
Hesperolinon drymarioides is a rare species of flowering plant in the flax family known by the common names drymary dwarf flax and drymaria-like western flax; it is named for its resemblance to genus "Drymaria".
 It is endemic to California, where it grows in the central inland North Coast Range.
 Most of the few known occurrences have been noted in Lake County.
 It is a plant of serpentine soils in chaparral and woodland ecosystems.
 This is a small annual herb growing a thin branching brown stem low to the ground or erect to about 20 centimeters in height.
 Leaves appear in whorls of four on the lower part of the stem and are dark reddish green with plentiful glandular hairs.
 Its flowers have light pink-veined white petals in a corolla about a centimeter across.
 Protruding stamens hold large yellow or purplish-white anthers.
Hesperolinon breweri is a rare species of flowering plant in the flax family known by the common names Brewer's dwarf flax and Brewer's western flax.
 It is endemic to California, where it is known from three counties in the San Francisco Bay Area.
 It is found in chaparral ecosystems, often on serpentine soils.
 This is an annual herb growing erect to a maximum height near centimeters.
 Its narrow, linear leaves are greenish to purplish in color.
 It produces dense inflorescences of flowers with glandular sepals and five bright yellow petals.
 The protruding stamens hold large orange-yellow anthers.
 This uncommon California endemic is threatened by development of its habitat but its current status is not known.
Hesperolinon tehamense is a rare species of flowering plant in the flax family known by the common names Tehama County western flax and Paskenta Grade dwarf flax.
 It is endemic to northern California, where it is known from only about ten occurrences, mostly within Tehama and Glenn Counties.
 Most of its habitat is on Bureau of Land Management lands and within the Mendocino National Forest, in chaparral ecosystems with serpentine soils.
 The plant produces thin, hairy stems up to 50 centimeters in maximum height with small, sparse linear leaves.
 The inflorescence bears several small flowers with pale to bright yellow notched petals just a few millimeters long.

Question: what does De Zoeker, Zaandam and Flax have in common?
Answer: linseed