112 THE SPORANGIUM DEFINED 
is not always maintained; that the elevation of the sporangia relatively to’ 
the surface of the part which bears them is variable ; that while a sporangial . 
wall is always present, the opening mechanisms are inconstant; that the 
tapettim is inconstant in occurrence and in origin; and that there is no 
general law underlying the segmentation of the sporogenous cell, or group 
of cells, so as to give it a constant hypodermal or other origin. What then 
remains as the fundamental conception of the sporangium in Vascular Plants ? 
Simply the spore-mother-cell or cells, together with the protective wall. The 
definition of a sporangium will then be this: Wherever there 1s found in 
Vascular Plants an isolated spore-mother-cell, or a connected group of them, or 
their products, this, together with its protective tissues, constitutes the essential of 
an individual sporangium. 
This definition is open to criticism, as indeed are all definitions of bodies 
represented in a large series of variable organisms. Still, it brings out the 
point that the essential feature of a sporangium .is the presence of one or 
more spore-mother-cells, but without reference to the detail of their 
production, or to the structure of the wall which covers them. It has its 
value in ridding the idea of the sporangium of its accidental accessories, 
and fixing the attention upon what is really essential. 
But it will perhaps be objected that a sporangium may still be a 
sporangium though it may contain no fertile cells; many imperfect 
structures may be quoted which have the form, position, and other details 
characteristic for the sporangia of the plant on which they are borne: 
they should thus to be ranked as sporangia. That is true; but as such 
bodies do not as a rule serve any useful purpose, it may be asserted that 
they would never have existed. independently of the fully formed sporangia 
of which they are the imperfect representatives. Such vestigial parts, being 
of secondary origin, need not disturb the conception of the sporangium as 
above defined. | * 
Finally, an important feature of the sporangium is to be seen in the 
fact that in so many cases the archesporium is not strictly circumscribed ; 
the sporogenous group has often ragged edges; in many of the Eusporangiate 
forms it does not arise from any single archesporial cell, or definite group 
of cells ; moreover, cells which are obviously sister-cells may not unfrequently 
be found to develop the one sterile, the other fertile. This suggests on 
the basis of structure that the fertile tract is a residuum left by advancing 
sterilisation, while the ragged and ill-defined limits point to the conclusion 
that the sterile and fertile tracts are closely related; in fact, that they had 
a common origin, and that the final condition represents the balance struck 
between sterile and fertile development. From the point of view of the 
hypothesis of progressive sterilisation such sporangia may, at least in the 
simplest cases, be regarded as islands of fertile tissue which have retained 
their spore-producing character. It will be seen later how far this view 
will have to be modified in the more complex and derivative cases, such 
as the Leptosporangiate Ferns. 
