oth mullein, wild carrot, wild parsnip, and burdock.

[Illustration: FIG. 58. A SPUD]

[Illustration: FIG. 59. HOUND'S TONGUE]

A third group of weeds consists of those that live for more than two
years. These weeds are usually most difficult to kill. They propagate by
means of running rootstocks as well as by seeds. Plants that live more
than two seasons are known as _perennials_ and include, for example,
many grasses, dock, Canada thistle, poison ivy, passion flower, horse
nettle, etc. There are many methods of destroying perennial weeds. They
may be dug entirely out and removed. Sometimes in small areas they may
be killed by crude sulphuric acid or may be starved by covering them
with boards or a straw stack or in some other convenient way. A method
that is very effective is to smother the weeds by a dense growth of
some other plant, for example, cowpeas or buckwheat. Cowpeas are to be
preferred, since they also enrich the soil by the nitrogen that the
root-tubercles gather.

[Illustration: FIG. 60. CANADA THISTLE]

Weeds do injury in numerous ways; they shade the crop, steal its
nourishment, and waste its moisture. Perhaps their only service is to
make lazy people till their crops.

=EXERCISE=

You should learn to know by name the twenty worst weeds of your
vicinity and to recognize their seeds. If there are any weeds you
are not able to recognize, send a sample of each to your state
experiment station. Make a collection, properly labeled, of weeds
and weed seeds for your school.

SECTION XXI. SEED PURITY AND VITALITY

Seeds produce plants. The difference between a large and a small yield
may depend upon the kind of plants we raise, and the kind of plant in
turn is dependent upon the seeds that we sow.

Two things are important in the selection of seeds--purity and vitality.
Seeds should be _pure_; that is, when sown they should produce no other
plant than the one that we wish to raise. They should be able to grow.
The ability of a seed to grow is termed its _vitality_. Good seed should
be nearly or quite pure and should possess high vitality. The vitality
of seeds is expressed as a per cent; for example, if 97 seeds out of 100
germinate, or sprout, the vitality is said to be 97. The older the seed
the less is its vitality, except in a few rare instances in which seeds
cannot germinate under two or three years.

Cucumber seeds may show 90 per cent vitality when they are