120 VARIATIONS IX NUMBER OF SPORANGIA 



plants it is possible to recognise certain methods of numerical change of 

 sporangia, now or previously operative : these may either lead to pro- 

 gressive increase in number, or to decrease in number. Under these two 

 heads the following table shows the several methods of change in number 

 of sporangia of which evidence has been found in living plants, but it is 

 possible that the table is not exhaustive : 



I. INCREASE IN NUMBER OF SPORANGIA. 



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

 (/') 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 (?) and (d). 



II. DECREASE IN NUMBER OF SPORANGIA. 



(f) By fusion of sporangia originally separate. 



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



(//) By reduction or arrest of apical or intercalary growth in parts bearing 



sporangia. 



(z) By fusion of parts which bear 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 (/z) and (/'). 



Each of these factors of variation will now be discussed, and examples 

 of them adduced. At the moment the object is only to recognise that 

 such modifications of number of sporangia are or have been operative 

 in actual cases, not to estimate the relative prevalence of any one : for 

 it is necessary first to distinguish the factors of the problem. 



FACTORS OF INCREAM . 



(a) Increase in number of sporangia by septation, resulting in a plurality 

 of loculi, where previously in the race the septa were absent, is shown 

 in the septate anthers of various families of Angiosperms (Onagraceae, 

 Mimoseae, Rhizophoreae, Myrsinaceae, Loranthaceae, etc.). The details 

 have already been sufficiently described in Chapter VII., p. 97. The 

 argument that septation has occurred is in many of these cases quite 

 conclusive : thus the plurilocular anthers of certain genera of the Onagraceae 



