50 Osborn. — Some Observations on Isoetes Drummondii , A.Br. 
a short distance. The bulk of the mucilage tissue is abaxial to the leaf- 
trace, though in some cases a little is developed on the adaxial side below 
the level of the sporangium (Fig. n). Mucilage also occurs in two or three 
of the outer cell-layers of the leaf-bearing cortex, but does not extend to 
any great depth in the stock. Below this sclerenchyma is developed. The 
process of mucilaginous thickening of the walls appears to go on until drought 
brings the vegetative activity of the plant to a close. 
The sporangia of Isoetes Drummondii vary in shape according to their 
position in the tightly packed rosette of leaf-bases. The outer sporangia 
are nearly circular in surface view, the inner elliptical, their length being 
2—4 times their breadth. In transverse section they have the usual boat- 
shaped section, i. e. their attachment to the sporophyll is along a relatively 
narrow strip of tissue. This tissue remains parenchymatous when the 
sporangium is fully ripe, but the wall of the sporangium undergoes 
considerable change. At the close of the vegetative season the wall 
consists of cuticle and epidermis only ; the two or three subepidermal layers, 
which persist in some other species, are lost in Isoetes Drummondii. The 
walls of the epidermal cells become altered as the sporangium matures: 
they thicken considerably, reducing the lumina of the cells, are dark brown 
in colour, and probably of a pecto-cellulose nature. The arrangement of 
the cells of the sporangium wall as seen in surface view (Fig. 12) is signifi- 
cant in connexion with the process of spore dispersal. Those on the flat 
top of the sporangium are irregular in shape (50-70 x 25-28 \ u) and form 
a close-fitting mosaic. Those composing the sides are rectangular (60-100 x 
rc-20 /x), regularly arranged with their longer walls running vertically. 
Moreover, in the side-walls of the sporangium certain narrow strands of 
parenchyma are found, radiating out from the line of attachment to the 
sporophyll. There may be as many as twenty-four or more such strands 
around the circumference of a sporangium. These thin-walled cells are 
obvious lines of weakness in an otherwise tough structure, and, as will be 
seen below, are functionally important as such in the process by which the 
spores are liberated. 
A further point of importance is the relation of the sporangium wall 
to the sporophyll. The thickened walls of the sporangium end sharply 
where they are attached to the parenchyma of the sporophyll, so that 
the whole way around the base of the sporangium there is a junction of 
tissues having very different mechanical properties. 
Process of Spore Liberation. 
The sporangia and sporophyll bases are in the condition just described 
at the close of the vegetative season. At its close there follows a period of 
