Pollen Development in Lactuca . 381 
stage of development, and measurements indicate that they are somewhat 
smaller than the nuclei of uninucleate cells in the corresponding stages of 
development. The cells, however, are of normal size or only slightly 
larger. The earliest stage at which binucleate mother-cells were found 
(and several were seen in this stage) is the presynaptic condition (Fig. 14). 
They were not traced farther back into the archesporium stage, and the 
manner of their origin i£ uncertain. Fig. 1 5 represents a mother-cell with 
its two nuclei in synizesis, while Figs. 16 and 17 represent postsynaptic 
and spireme stages. The latter represents the latest stage in which 
binucleate mother-cells were found. Theoretically, if such a mother-cell 
completed the meiotic divisions it should give rise to eight pollen grains, 
but there is no evidence that this happens. It is possible that such cells 
break down before the heterotype mitosis. If the meiotic processes were 
carried through, it is conceivable that pollen grains with two nuclei or 
a diploid nucleus or some other aberrant condition might result. Possibly 
some of the aberrant pollen tetrads already mentioned originated from 
binucleate mother-cells. 
Regarding the origin of the binucleate condition in presynaptic cells, 
we have no absolutely conclusive evidence, but the indications are that it 
arises through the break-down or incomplete formation of a cell membrane 
between the two cells. 
In the recent literature, a number of cases of binucleate cells in somatic 
tissue have been described. Their occurrence is so frequent as to indicate 
that co-ordination between nuclear division and cell-wall formation is by 
no means universal in growing tissues. The literature of the subject is 
summarized by Beer and Arber (1920), who have also contributed (1915) 
many of the recent results. Cells with two or sometimes more nuclei have 
been observed, particularly in the pith and cortex of the growing region 
of stems in a variety of plants, such as Asparagus (where such cells appear 
to be particularly frequent), Eremurus , Helianthus , Monstera , Hemero- 
callis , Elodea , Stratiotes , &c. They found binucleate cells in the growing 
region of the stem in 177 species belonging to the Pteridophytes, Gymno- 
sperms, and Angiosperms. Polynucleate cells were also found in roots and 
in the mesophyll of leaves. The binucleate or multinucleate condition was 
found to arise invariably through mitotic rather than amitotic division. 
The authors (Beer and Arber, 1919 ; Arber, 1920) have also shown the 
manner of origin of the binucleate condition. A cell-wall is not laid down 
in connexion with the cell-plate of the spindle, but the spindle fibres and 
associated cytoplasm become transformed into an enlarging hollow sphere 
which expands until it encloses the daughter nuclei and has been called 
a phragmosphere. The two nuclei so formed in the same cell may differ 
somewhat in their later behaviour. One may divide without the other, 
and this division may be followed by cell-wall formation. In other cases 
