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CYTOLOGY
THE discovery of nuclei in smuts was delayed by their small size, which also
accounts for our incomplete picture of their division and behaviour in the lifecycle.
In many published figures the nuclei are little more than black dots, and
the magnification is often omitted. ^Estimating from Rawitscher's plates,
which are among the best, it seems that the resting nucleus in the ripe spore of
Tilletia caries is about 3 X 5 /i in diameter. After 47 hours in water, when in
preparation for division, the size increases to 5 X 7 /i. Nuclei passing out into the
promyceUum are considerably smaller, 1-2 jit only, and this may be taken as the
approximate size of haploid nuclei, up to the moment of fusion in the ripe resting
spore (Rawitscher, 1922).
The discovery of this fusion was due to the work of Dangeard (1893, 1894 b)
who studied sporogenesis and observed the change from the binucleate to the
uninucleate condition in seven species of smuts. Speaking of Doassansia
alismatis Dangeard describes the nucleus of the ripe spore as ' nucleole, charge
de chromatine et reconvert d'une membrane nucleaire, a peine observe-t-on
quelques fins trabecules de protoplasma qui rayonnent vers la parol; tout le reste
est forme d'une substance oleagineuse qui donne aux oospores vivantes leur
aspect blanc et refringent'. Some of the figures in Dangeard's paper suggest
dividing nuclei. Harper (1899), employing the triple stain, described in the
phraseology of his day the resting nucleus in a promyceHum of Ustilago
scabiosae as showing 'a sharply differeritiated, blue stained chromatin net Ijring
in a clear nuclear sap, a red stained nucleole and a surrounding membrane'. He
was the first to describe nuclear division in a smut. 'The equatorial plate stage
is very distinct and shows a sharply pointed bipolar spindle, whose fibres end in
deep staining granules at the poles. No polar radiations at this stage have been
observed. The chromosomes are rather densely massed at the equator and are
probably eight or ten in number.' The figure of U. scabiosae, to which reference is
made, might be interpreted as an early anaphase showing the separation of
three bivalents. Two drawings of nuclear division in sporidia of U. violacea show
chromosomes arranged on a spindle, but again the number cannot be exactly
stated. Harper's work indicated that the nuclei of the smut fungi divide' in a
manner similar to those of the higher plants, and showed also that the copulation
of sporidia was not immediately followed by the fusion of their nuclei.
Rawitscher (1912, 1914) figured resting nuclei in his papers on the origin of
the binucleate condition in some cereal smuts, and in later work (1922) dividing
nuclei with intranuclear spindles and chromosomes. He states that in a ripe
uninucleate spore of Tilletia caries the nucleus lies near the wall and in it one dr
perhaps two nuclear bodies can be recognized. After 40 hours in water a muchenlarged
nucleus is seen in a condition similar to synapsis. The content is very
refractive and, in addition to the globular large nucleolus, a smaller one may be
present. The chromatin is found in one, sometimes in two nets at the wall of the
nucleus (Figs. 2 and 3). Spores fixed some hours later showed the prophase
(spireme threads), and in some nuclei four stainable bodies could be recognized.
In material fixed after 46 hours some spores already contained four nuclei, but
stages of nuclear division are rare. Fig. 7, which shows a nucleus with two