Pollen Development in Lactnca . 371 
difference between the synizesis and postsynizesis measurements in L. 
Scariola as regards size of both cells and nuclei is probably not signi- 
ficant, and this would appear to indicate that cell growth ceases earlier 
in this species than in the lettuce. 
Table I. Dimensions of Cells and Nuclei in Lactuca . 
Lettuce — Rogue. 
L. Scariola. 
Average Diameter. 
Average Diameter 
mm. 
mm. 
Cell. 
Resting nucleus 
19.7 
1 7*7 
Beginning of synizesis 
20-3 
17.8 
Synizesis 
21-8 
23 - 4 * 
Postsynizesis 
26 
22.5 
Nucleus. 
Resting stage 
TI -4 
IO.I 
Beginning of Synizesis 
12-8 
13-4 
Synizesis 
14-2 
M -9 
Postsynizesis 
15.2 
14.6 
Reticulum. 
Early in synizesis 
7-7 
7-4 
Synizesis 
6-5 
7-4 
That in synizesis the reticulum undergoes contraction to approxi- 
mately half the diameter of the nucleus is shown by comparison of the 
last two lines of the table with the preceding lines. Needless to say, 
all the nuclear measurements were taken from nuclei which had not been 
cut, so that the whole of the contracting reticulum or synaptic knot was 
present. With nuclei of such small size this is easily done when the 
sections are 10 // thick. 
Hitherto, synizesis has only been reported from spore mother-cells 
in plants and in the primary spermatocytes and oocytes of animals. This 
has emphasized the purely morphological or developmental aspects of the 
process. It evidently represents a unique and critical stage in the life 
cycle of any sexually reproduced organism, having been observed even 
in such low organisms as the Myxomycetes (Olive, 1907 ). But like every 
other developmental stage, it must be produced by antecedent physio- 
logical conditions which bring it about. These conditions usually arise 
only in spore mother-cells. But a peculiarity of Lactuca , as already pointed 
out, is the presence of a variety of intergrades between pollen mother-cells 
and tapetal cells. The presence of such intergrades is believed to account 
for the occurrence of synaptic phenomena in cells which are essentially 
tapetal cells. 
Fig. 74 shows a binucleate tapetal cell with its nuclei in synizesis. 
That it is a tapetal cell and not a binucleate microspore mother-cell, such as 
Fig. 15, is shown by its position. It forms a part of the tapetal lining of 
the loculus, while the pollen mother-cells are in the postsynaptic spireme 
stage. The adjacent tapetal cells do not show this condition, except one 
trinucleate cell (Fig. 82) on the opposite side of the loculus. The central 
