1901 | DEVELOPMENT OF THE POLLEN 327. 
archesporium to the pollen mother cell. The first indication of 
the formation of sporangia is the increase in length of the hypo- 
dermal cells on the inner side of the stamen on each side of its 
median line, thus forming the archesporium (jig. r). While 
both sporangia are formed from the same continuous layer, they 
are separate from their beginning ; and there are no indications 
of four. The tapetum on the dorsal side of the sporangium is 
formed from the third layer of cells—the one next the arche- 
sporium. The archesporial cells elongate at right angles to the 
inner surface of the stamen and divide transversely, resulting in 
an inner primary sporogenous, and an outer primary wall layer. 
The beginning of this division is shown in fig. 7. 2 By transverse 
divisions of the primary wall layer there arise four wall layers 
( fig. 3), the inner one or two of which form the tapetum. The 
tapetal nature of these cells is already evident in their large 
nuclei and nucleoli, and their deeply staining contents, before 
they have completed their division ( fig. ¢).- In some places the 
tapetum is composed of one layer, in others of two. Where 
there are two the outer is often composed of flattened, the inner 
of isodiametric cells (figs. 6 and rg). While the primary wall 
layer forms its four layers, the plate of cells dorsal to the 
Sporangium also divides and takes on its tapetal character. 
The primary sporogenous cells contain large vacuoles, while 
their nuclei are near the middle (fig. 4) ; but even at this stage 
of development they grade into ordinary vegetative cells at the 
outer edge of the sporangium, so that it is difficult to determine 
just where they cease to be sporogenous cells ( fig. 3). Enough 
stages were seen in the other species mentioned to warrant the 
conclusion that the development of the pollen in them is the 
Same in general as in A. Cornuti. 
From this point the history was most completely followed 
in A. tuberosa. The primary sporogenous cells become the 
Spore mother cells without further division. This is indicated 
by the nature of the nuclei and by the form and number of the 
chromosomes. The size of the nucleus increases, and the chro- 
Matin collects in granular tangled threads, which together with 
