Pollen Development in Lactucct . 
379 
Formation of Pollen Tetrads and Pollen Grains. 
When the reduction divisions are completed, the first evidence of 
further development in the pollen mother-cell is the beginning of constriction 
of the cytoplasm at four points placed at equal intervals on its periphery 
and within the mother-cell wall (Fig. 66). The interval between the 
cytoplasm and the cell-wall appears to be filled with pale-staining material 
whose nature has not been investigated. The constrictions of the cytoplasm 
become progressively deeper until they meet in the centre (Fig. 67 ) and 
finally cut up the cytoplasm into four separate masses, the microspores 
(Fig. 68 ). As the figures show, this process of constriction of the cytoplasm 
may take place in the absence of spindle fibres, or it may cut across them 
before they have disappeared, but in no case were cell plates observed to 
be laid down on the spindles, the whole process taking place independently 
of such a structure. Fig. 69 shows a somewhat abnormal tetrad as regards 
the shape of the mother-cell, and also the way the cytoplasm has been 
divided up. The mother-cell wall has disappeared and one of the walls 
has failed to appear, making a central cell with two pollen nuclei. Fusion 
of these nuclei would give a diploid pollen cell. 
It has usually been regarded as an important distinction between the 
cells of higher plants and animals that whereas in the latter the cytoplasm 
is segmented after nuclear division by furrowing or cleavage, in plants the 
new cell-wall is laid down on the spindle. Since Strasburger described the 
latter method of cell-wall formation in various plants and different tissues, 
it has usually been assumed to be universal in higher plants. Like so many 
other distinctions drawn between plants and animals, this is now found to 
be a general but by no means universal difference. Farr (1916), who refers 
to the early literature, has described in detail the division of pollen mother- 
cells in Nicotiana by furrowing, exactly as observed in Lactuca. He has 
observed the process both in living and fixed material, and has also 
described it (1918) in Magnolia pollen mother-cells. The same method of 
furrowing has recently been described by Wanda K. Farr (1920) in Cobaea 
scandens. There is no doubt that it will be found to occur in a large 
variety of plants, though by no means to the exclusion of the older method, 
which occurs, for example, in Oenothera (Gates, 1907) and has recently been 
described in detail (Yamaha, 1920) in Psilotum . Farr (1916) attempts 
a physical explanation of the furrowing as a result of the accumulation of 
electrical charges on surfaces and membranes, and the resulting attractions 
and repulsions which develop. 
The young pollen grains within the mother-cell wall begin at once to 
alter their shape, each one becoming approximately heptagonal in cross 
section. This is done while the young pollen grain is surrounded only by 
a plasmatic membrane, and the change in shape can only be a result of the 
