320 LYCOPODIALES 
sporangium (Fig. 165 B). The potential sporogenous tissue thus produced, 
after successive sub-divisions, forms a very considerable sheet of tissue, 
several cells in thickness. Of this, however, only a portion develops into : 
spores: in the case of a microsporangium certain tracts of cells of this 
tissue assume dense protoplasm, and the cells, ultimately separating from one 
another, undergo the tetrad-division, producing 
microspores (Fig. 165 c, D); but other tracts of 
cells, neither showing any regular outline or 
arrangement, nor referable in origin to pre- 
determined cells of the genetic tissue, become 
less densely protoplasmic, and form the sterile 
trabeculae: a tapetal tissue invests the fertile 
tracts: it is derived partly from the innermost 
layer of the sporangial wall, as in Lycopodium, 
partly from the superficial cells of the trabeculae. 
A similar differentiation of the potentially sporo- 
genous tissue is found also in the megasporangia, 
the early stages of which are quite indistinguish- 
able from those of the microsporangia; but in 
the former a relatively smaller number of cells, 
usually lying isolated in the potential sporo- 
genous tissue, and distributed with no constant 
relation to their ultimate parent cells, enlarge 
and divide to form the megaspores (Fig. 166). 
As there is no opening mechanism in the 
submerged sporangia of Jsoeées, no basis for 
comparison is yielded from that source. The 
study of the development in /sve¢es thus leads 
clearly to the conclusion that there has been 
a differentiation, within the sporangia, of tissues 
at first of uniform character: that part of the 
Peas eee Thecell snkea ~~ potential sporogenous tissue remains fertile, but 
(rr) Fe te only fertile spore-motner” a large proportion in the microsporangium, and 
tive divisions, including the cell (@) 4 still larger proportion in the megasporangium, 
as shown by other sections of the 
series. Thus sterilisation affects the ig diverted to other uses, and remains sterile. 
large majority of the cells of the 
Pie stay X74 After” As regards the origin of the potential. sporo- 
genous tissue, and the form and position of the 
sporangium, there is clear correspondence to the Lycopod-type, and 
especially to those forms with the more bulky sporangia: in fact if we 
imagine a heterosporous Lycopod, with its sporangium widened out radially 
along the leaf-surface and its enlarged sporogenous tissue partly sterilised 
so as to form trabeculae, the result would be practically what is seen in 
Lsoetes. 
A study of the sporangia of the fossil Lycopods is a necessary adjunct 
to that of the modern forms, though the usual absence of developmental 
Fic. 166. 
