H2 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 

 tapetum 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 is found in 

 Vascular Plants an isolated spore-motlier-ccll, 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. 



