﻿2 86 BOTANICAL GAZETTE [APRIL 



the general application of a definite term to those ultimate parent cells 

 which the analysis discloses has no scientific meaning, beyond the 

 statement of the histogenic fact. 



Further, it is shown that the tapetum is not a morphological con- 

 stant, but varies both in occurrence and origin; that even the individu- 

 ality 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 7nother cell, or connected group of 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 sterili- 

 zation 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; ip) by formation of 

 new sporangia, or of new spore-bearing organs, which may be in addi- 

 tion to or interpolated between those typically present; {c) by con- 

 tinued 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 (^) and {d)^ 



II. Decrease in number of sporangia, — (/) By fusion of sporang 

 originally separate; {g) by abortion, partial or complete, of sporangia; 

 (/z) by reduction or arrest of apical or intercalary growth in parts 

 bearing sporangia; {J) by fusion of parts which bear the sporangia or 

 arrest of their branchings; (y) indirectly, by suppression of branch- 

 ings in the non-sporangial region, resulting in decreased number of 

 sporangial shoots; this is closely related to (//) 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 refmU- 



o^ia 



