190:5.] 



Morphology of Spore-producing Members. 



259 



but varies both in occurrence and origin ; that even the individuality 

 of the sporangium is not always maintained. All that remains then 

 as the fundamental conception of the sporangium in vascular plants 

 is the spore-mother-cell, or cells, and the tissue which covers them in, 

 for such cells are always produced internally. The definition of the 

 sporangium may then be given thus : " Wherever we find in vascular 

 plants a single spore-mother-cell, or connected group of them, or their products, 

 this with its protective tissues constitutes the essential of an individual spor- 

 angium." 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 in 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 progressive increase 

 and decrease : — 



I. — Increase in Number of Sporangia. 



(a.) By septation, with or without rounding off of the individual 

 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 sporangial shoots ; this is closely 



related to (c) and (d). 



II. — Decrease in Number of Sporangia. 



(/.) 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. 



( /.) Indirectly, by suppression of branchings in the non-sporangial 

 region, resulting in decreased number of sporangial 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) 



u 2 



