Nature Bulletin No. 469-A   November 4, 1972
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

****:WEED SEEDS

The study of weed seeds can be fascinating, educational, and an 
inspiration for artistic designs -- especially if you use a good 
magnifying glass or hand lens. People wonder why weeds are so 
aggressive; why they succeed where desirable plants fail; why they 
thrive in spite of droughts, grass fires, mowing and cultivation. One 
answer, in the case of most weeds, can be found in the seeds that they 
produce.

Some weed seeds have such a hard durable outer coat that they are 
unharmed by an ordinary grass fire. If eaten by a bird or a grazing 
animal, they are not digested in its stomach and are widely distributed. 
Some kinds have such vitality that they have been known to sprout and 
grow when brought to the surface after being buried deeply for many 
years. Many weed seeds have interesting appendages which cause them 
to be carried long distances by winds or which enable them to cling to 
passing animals.

The seeds of some weeds are enclosed in small "nutlets". Beggar-lice, 
nutlets of one of the Stickweeds, are covered with clusters of from 3 to 
5 fine barbs resembling the claws on the foot of a housefly. On another 
kind the clusters are star-shaped like the end of a dentist's drill. A 
Puncture Vine nutlet has spines arranged so that, no matter how it lies, 
one will be pointing upward -- exactly like the French caltrop and the 
Roman tribulus used to stop an enemy's cavalry. Similarly, a Sandbur 
will cause painful injury to a barefoot boy, the paws of dogs and cats, or 
even puncture a bicycle tire.

The Burdock and the Cocklebur produce quantities of burs with hooked 
spines arranged in ornate but very effective patterns. The coats of dogs 
or other animals, and the tails of horses and cows, frequently become 
matted with these pesky burs which, when they are pulled off, spill out 
large numbers of small seeds. The cocklebur is an example of plants 
that produce more than one type of seed; some of its burs germinate the 
following spring, others in two years, and so on.

There are weed seeds armed with slender pointed bristles, called 
"awns", that expand and contract according to the amount of moisture 
encountered. Thus, the Wild Oat seed has two awns which enable it to 
creep along the ground until it can enter some crevice. Porcupine or 
Needle Grass seed has two which twist like an auger and bore into the 
soil.

Some weed seeds have odd shapes which are responsible for the 
common name of the parent plants. Those of the Moonseed vine are 
crescent-shaped like a new moon. The Spanish Needle has a long 
slender seed armed at the broader end with three prongs and each of 
these has a sharp curved hook. The Bootjacks or Beggar-ticks, with two 
such awns, resemble that old-time implement used on farms and 
ranches. Docks, Smartweeds and Sorrels have wedge-shaped seeds 
which help them to easily work into soil openings.

Some weeds, such as Dandelion, Wild Lettuce and the Thistles, have 
tufts or "parasols" of fine hairs attached to their seeds, and these serve 
like parachutes which are carried long distances by winds. Those 
parachutes are interesting. The Seed of the Goatsbeard, now a roadside 
pest, has one with a lacy arrangement of silky threads which travels 
daily until it passes over a moist area. Then the parachute, called a 
"pappus", closes and the seed drops to the ground.

A race of plants depends upon its seeds -- each one a masterpiece of 
arrangement and construction.