1899 ] LIFE-HISTORY OF LEMNA MiNOR 49 
determined by further work. The question certainly deserves 
careful investigation. 
Following the stage last cited (fig. 76), the archesporial cells 
continue to divide until two or three times the number of cells 
are developed. At about this time there appears immediately 
beneath the epidermis a layer of cells which by their reactions 
to stains are clearly differentiated from the cells beneath 
(fig. 17). Their nuclei stain more deeply, and the cytoplasm 
less deeply than the nuclei and cytoplasm of the cells beneath. 
This is the primary tapetal layer, while the cells below are the 
sporogenous cells, or probably by this time they are the micro- 
spore mother cells. The primary tapetal layer, therefore, is not 
cut off from the archesporium immediately following the appear- 
ance of the latter as a primitive layer, but after considerable 
masses of archesporial tissue are formed and segregated. It is 
not a primary but a secondary differentiation. 
Between the mother cells and the epidermis in older loculi there 
are three or four layers of tapetal cells (jfig.78). The number of 
these layers is not 
necessarily regular 
for all the parts of 
a given loculus, as 
is shown in the fig- 
ure just cited. | 
have not been able 
to decide definitely 
upon the origin of 
the tapetum. In 
some preparations Fic. 17. A single loculus; tapetal layer differentiated 
the relative PoOSi- about sporogenous cells. < 1375. 
tions of the cells a ee — ek 
indicate that the which shows several wall layers and the tapetum. é 
tapetum has been cut off from the sporogenous cells, while in 
other cases it seems equally clear that it has come from the 
wall layers. It will be borne in mind that in the sporangia of 
the pteridophytes the tapetum is cut off from the sporogenous 
