Although thistle seeds have a prominent pappus 1 , the relatively large seed size limits the distance seeds are carried in the wind. During peak flowering of musk thistle large ‘clouds’ of pappus can be seen floating gently in the breeze. While giving the impression that the seed are being carried long distances, in most instances these high flying remnants of the musk thistle are detached pappus with no seed attached. While some seed may be carried long distances from the source, the majority of seed falls relatively close to the mother plant. The distance seed is carried obviously is strongly influenced by wind speed. Research at Virginia Tech found that under calm conditions 95% of the seed fell within 30 ft of the mother plant (Figure 1). With wind speeds of 12 MPH, 87% of the seed fell within 75 ft of the mother plant. Approximately 4% of the seed was carried more than 200 ft.
Feldman and Lewis. 1990. Weed Research 30:161-169.
Table 1. Emergence of plumeless thistle seedlings in relation to mother plant.
Many important weed species have adaptations for efficient dispersal of their seed. Although thistle seed are adapted to wind dispersal, their relatively large seed size limits long distance movement under most conditions. Musk thistle plants directly across a fence line can serve as a source of infestation, but as the distance increases to more that 100-200 feet the likelihood of seed traversing the distance is greatly diminished. Dandelion and horseweed, two additional weeds with wind-blown seed, have much smaller seed than thistles, and therefore are more likely to be carried long distances by wind currents.
Researchers in Argentina planted plumeless thistle in fields previously not infested with the species, and then determined the emergence of thistle seedlings in relation to distance from the mother plant. More than 95% of the seedlings emerged within 2 ft of the mother plant, indicating that most seed fell directly to the ground.
T he remarkable Protea Family (Proteaceae) of Australia contains some truly amazing genera with winged seeds, including Banksia and Hakea . Although they are flowering plants, banksias produce a dense flower cluster (inflorescence) that gives rise to a cone-like structure containing many woody carpels. Each carpel bears 2 winged seeds and the entire cone-like structure superficially resembles a pine cone. In fact, some banksias release their seeds following fire and even resprout from subterranean lignotubers like chaparral shrubs.
Probably the best way to appreciate the relative hardness of different woods is the concept of “specific gravity,” a numerical scale based on 1.0 for pure water. Without getting too mathematical, the specific gravity of a substance can easily be calculated by dividing its density (in grams per cubic centimeter) by the density of pure water (one gram per cubic centimeter). The brilliant Greek mathematician and inventor Archimedes discovered over 2,100 years ago that a body in water is buoyed up by a force equal to weight of the water displaced. Archimedes reportedly came upon this discovery in his bathtub, and ran out into the street without his clothing shouting “Eureka, I have found it.” Since one gram of pure water occupies a volume of one cubic centimeter, anything having a specific gravity greater than 1.0 will sink in pure water. The principles of buoyancy and specific gravity are utilized in many ways, from scuba diving and chemistry to the hardness of dry, seasoned wood. Some of the heaviest hardwood trees and shrubs of the United States have specific gravities between 0.80 and 0.95; including shagbark hickory ( Carya ovata ), persimmon ( Diospyros virginiana ) and ironwood ( Ostrya virginiana ) of the eastern states, and canyon live oak ( Quercus chrysolepis ), Engelmann oak ( Q. engelmannii ), hollyleaf cherry ( Prunus ilicifolia ) and Santa Cruz Island ironwood ( Lyonothamnus floribundus ssp. asplenifolius ) of southern California. Although some of these trees are called ironwoods, their dense, dry wood will still float in water. Since the pure cell wall material (lignin and cellulose)) of wood has a density of about 1.5 grams per cubic centimeter, even the world’s heaviest hardwoods generally have specific gravities less than 1.5 due to tiny pores (lumens) within the cell walls. True ironwoods include trees and shrubs with dry, seasoned woods that actually sink in water, with specific gravities greater than 1.0. They include lignum vitae ( Guaicum officinale , 1.37); quebracho ( Schinopsis balansae , 1.28); pau d’arco ( Tabebuia serratifolia , 1.20); knob-thorn ( Acacia pallens , 1.19); desert ironwood ( Olneya tesota , 1.15); and ebony ( Diospyros ebenum , 1.12). To appreciate the weight of these hardwoods, compare them with tropical American balsa ( Ochroma pyramidale ), one of the softest and lightest woods with a specific gravity of only 0.17.
A population explosion of western salsify( Tragopogon dubius ) near Mono Lake, on the east side of the Sierra Nevada of Central California. This ubiquitous species is actually native to Europe and Asia.
H elicopters (also called Whirlybirds) include seeds or one-seeded fruits (samaras) with a rigid or membranous wing at one end. The wing typically has a slight pitch (like a propeller or fan blade), causing the seed to spin as it falls. Depending on the wind velocity and distance above the ground, helicopter seeds can be carried considerable distances away from the parent plant. The spinning action is similar to auto-rotation in helicopters, when a helicopter “slowly” descends after a power loss.
O ther South American species of Tabebuia are also referred to as pau d’arco, including the pink-flowered T. impetiginosa and T. avellanedae . According to The New York Botanical Garden Encyclopedia of Horticulture Volume 10, 1982, T. avellanedae is a synonym for T. impetiginosa , and T. ipe “is so closely similar to T. impetiginosa that it can scarcely be more than a variety of that species.” These attractive pink-flowered species are commonly used as landscape trees in temperate regions.
O ne fuzzy brown cattail spike may contain a million tiny seeds. Each seed has a tuft of silky white hairs and is small enough to pass through the “eye” of an ordinary sewing needle. They are shed in clouds of white fluff and float through the air like miniature parachutes. A cattail marsh covering one acre may produce a trillion seeds, more than 200 times the number of people in the world! The fluffy seeds have been used for waterproof insulation and the buoyant filling of life jackets. In addition, each plant produces billions of wind-borne pollen grains; in fact, so much pollen that it was used as flour by North American Indians and made into bread. Cottonwoods and willows also produce masses of seeds, each with a tuft of soft, white hairs. Since they are dioecious, with pollen-bearing male and seed-bearing female trees in the population, only female trees produce the actual cotton. During late spring and summer in the western United States, the cottony fluff from cottonwoods resembles newly fallen snow. Because the wind-blown fluff can be quite messy in cultivated parks and gardens, male trees are generally planted. The discriminatory label of “cottonless cottonwood” refers to a male tree.
Helicopters: A. Box Elder ( Acer negundo , Aceraceae); C. Big-Leaf Maple ( Acer macrophyllum , Aceraceae); E: Evergreen Ash ( Fraxinus uhdei , Oleaceae); F. Tipu Tree ( Tipuana tipu , Fabaceae).