Morphologie und Teratologie. 411 



group ot them, or their products, this with its protective tissues 

 constitutes the essential of an individual sporangium". From 

 the point of view of a theory of Sterilisation such sporangia 

 may, at least in the simplest cases, be regarded as islands of 

 fertile tissue which have retained their spore-producing character, 

 while the surrounding tissues have been diverted to other uses. 

 It will be seen later how far this view will have to be modified 

 n the more complex cases. 



In a second section of the Memoir the variations in number 

 of sporangia in vascular plants are discussed; the methods of 

 Variation may be tabulated as follows, under the heads of pro- 

 gressive increase and decrease: 



I. — Increase in Number of Sporangia. 



a) By septation, with or without rounding off of the indi- 

 vidual sporangia. 



b) By formation of new sporangia, or of new spore-bearing 

 organs, which may be in addition to, or interpolated 

 between those typically present. 



c) By continued apical, or intercalary growth of the parts 

 bearing the sporangia. 



d) By branching of the parts bearing the sporangia. 



e) Indirectly, by branchings in the non-sporangial region 

 resulting in an increased number of sporangia! shoots; 

 this is closely related to c) and d). 



II. Decrease in Number of Sporangia. 



f) By fusion of sporangia originally separate. 



g) By abortion, partial or complete, of sporangia. 



h) By reduction or arrest of apical or intercalary growth in 



parts bearing sporangia. 

 i) By fusion of parts which bear the sporangia or arrest of 



their branchings. 

 j) Indirectly, by suppression of branchings in the non-spo- 

 rangial region, resulting in decreased number of spo- 

 rangial shoots; this is closely related to h) and i). 



We are justified in assuming that (subject to the possibility of 

 other factors having been operative of which we are yet unaware) 

 the condition of any polysporangiate sporophyte as we see it is the 

 resultant of modifications such as these, operative during its descent. 



The problem will, therefore, be in each case to assign its proper 

 place in the history to any or each of these factors. 



It is pointed out that in homosporous types^ which are certainly the 

 more primitive, the larger the number of spores the better the chance of 

 survival, and hence, other things being equal, increasing number of 

 spores and of sporangia may be anticipated ; but in the heterosporous 

 types reduction in number both of spores and of sporangia is frequent. 

 The former will accordingly illustrate more faithfully than the hetero- 

 sporous forms the story of the increase of complexity of spore-producing 

 parts. The general method put in practice here is to regard homosporous 

 forms as in the Upgrade of their evolution, as regards their spore pro- 

 ducing organs, unless there is clear evidence to the contrary. The onus 

 probandi lies rather with those who assume reduction to have taken 

 place in them. 



