PLANT NEMATODES THE GROWER SHOULD KNOW 23

best to begin with those types capable of attacking the growing roots
of plants. Certain species, of course, are of far greater economic im-
portance than others. It is with these species that we will first deal.

 THE ROOT-KNOT NEMATODE
 Root-knot nematodes (Meloidogyne spp., formerly Heterodera marioni
.[Cornu] Goodey) are considered the most important of this group.
Their extended distribution through the tropics, sub-tropics, and tem-
,perate regions and their occurrence in greenhouses everywhere, make
them one of the most widely distributed and common agricultural pests.
They have long been known in Florida. Neal states that the disease, as
such, was known to occur here as early as 1805, although the causitive
organism, the nematode, was not discovered until 1879, and was first
definitely established as occurring in Florida in 1889. While root-knot
doubtless is now present in all sections of the State it is particularly
bad in those areas where sandy and peaty soils prevail, where it causes
damage in various ways that may well be estimated at several million
dollars annually. It also has been reported to occur in almost every State
of the Continental U.S.A. Although these organisms were formerly
thought incapable of surviving the winter in the northern part of our
country, this is a misconception, for experiments and observations show
some species fully capable of withstanding extreme cold when in the
soil, and of surviving temperatures as low as 100F. if exposed. Root-
knot does not cause such extensive damage farther north, however, as
it produces under warmer climatic conditions apparently because north-
ern summers are short and cool, thus preventing the development of
more than one or two generations.
 In the description of the life cycle of the root-knot nematodes we
may best begin with the preparasitic larva (Fig. 12C) after it has hatched
from the egg (Fig. 12 A & B) and migrated through the soil until it
has reached a root tip, where it makes its entry. After migrating to the
axial cylinder of the root, it becomes sedentary, (Fig. 13A) and, with
its short buccal stylet, injects the secretion of its oesophageal or salivary
glands into the tissue of the roots. Thereupon the cells of the root begin
to form from three to five so-called giant cells at the injection point
around the oral opening of the nematode (Fig. 13 B-D). These cells
are also called nectarial cells, from which the nematode absorbs its
food during its entire life. With the intake of food the larva begins to
swell rapidly, first becoming sausage-shaped (Figs. 12 D and 13 B-D)
and then growing to a pear-shaped, whitish body (Figs. 14, 15 & 16).
At this stage of development it may be so large in some instances that
it can be seen with the naked eye (Fig. 17). Under favorable conditions
this development may take three to four weeks, but a much longer time
is often required, particularly when the temperature is suboptimal. The
full-grown female produces eggs (Fig. 18 B) which are deposited in a
yellowish-brown, jelly-like substance which flows from the female geni-