_ Ig00} SPOROPHYLLS AND SPORANGIA OF ISOETES 247 
and 50, the cells of this layer multiply rapidly. They are fre- 
quently found in mitotic division, with the axis of the spindle 
always perpendicular to the surface of the trabecule or sporan- 
gium wall. Divisions may still go on here after the spore 
mother cells have reached maturity, and the changes of the 
trabecule are nearly complete. In this way the tapetal cells 
become very numerous, but reduced in size. They form but a 
single layer except in limited areas, where a doubling may some- 
times occur. | 
At first the tapetum is not deeply stained (fig. 50), but as 
the spore mother cells prepare for their tetrad division, the 
tapetal contents increase in density, and they continue to do so 
until they surpass young spores in this respect. 
From what has been said, and from figs. 47, etc., it will be 
understood that the tapetum completely invests the trabecule 
and sporangium wall, forming a lining layer everywhere between 
the spore mother cells and the sterile regions. Itisa persistent 
layer, and in this respect is to be contrasted with that of most 
ferns and angiosperms. In these latter the walls of the tapetum 
break down and are dissolved, the cells become disorganized, 
and their materials, mingling with the other contents of the 
sporangium, are used to nourish the mother cells or young spores. 
In Isoetes, however, as in Lycopodium and Selaginella, no such 
disorganization of the tapetum occurs. Its cells do not fall 
apart and its walls are not absorbed. In old sporangia it is still 
recognizable, though often its contents have been lost and the 
walls are pushed nearly together. 
Probably the tapetum can best be regarded as a gland or 
layer of glandular cells. If so, the manner of action in a per- 
sistent tapetum, such as that of Isoetes, Lycopodium, and Selagi- 
nella, must be quite different from what it is in a tapetum which 
is regularly disosganized and absorbed. In the one case the 
nutrient substances secreted by the cells must be passed on 
through the walls into the cavity in which the young cells are 
growing. In the other case there can be little or no passing of 
nutrient substances through the walls, but at the proper time the 
