THE POLY-SPORAXGIATE STATE 249 



also probably primitive and certainly early, the appendages are larger, and 

 the sporangia more removed from the axis ; and in proportion as this is 

 so their number is less precise. But even where the appendages are 

 largest, as in Ferns, or Ophioglossaceae, the relation of leaf to axis remains 

 essentially the same. 



The variations of number of sporangia actually seen as effective in 

 Vascular Plants have been discussed in Chapter X., in which methods of 

 increase are separated from those of decrease. Both of these are liable 

 to be disguised by the swamping effect of continued apical growth, and of 

 branching of axes and appendages, which are so prevalent in Vascular 

 Plants. But, putting these more obvious sources of numerical change of 

 sporangia aside, there are others which have also been effective, and have 

 probably played an important part in evolution. As factors of increase in 

 number of sporangia septation and interpolation are to be recognised. 

 The former of these has probably been underestimated hitherto in its 

 evolutionary effect : numerous synangial bodies in Pteridophytes are com- 

 patible with it, and each must be considered on its merits ; moreover, 

 septation is demonstrated to have actually occurred in the anthers of a 

 number of Angiosperms. Interpolation of new sporangia among those 

 previously present, on the other hand, has hitherto been overestimated : 

 in certain of the simplest forms, and particularly in the Lycopods, it is 

 non-existent : it is more prominent in larger-leaved forms, where sporangia 

 are indefinite in number, such as the Ferns, and it has played an 

 important part among the later Polypodiaceae ; but no clear case of it 

 is known among Palaeozoic Plants. It is held as a relatively late mode 

 of increase, initiated as a secondary phenomenon, and it cannot be 

 assumed to have been of general occurrence in the course of descent. 



Among the factors of decrease in number of sporangia the arrest of 

 apical growth in axes, or in appendages, has probably been one of the 

 most effective, and especially so in the later types of development ; but 

 as this, where operative, would leave no trace behind of what had actually 

 occurred, it is liable to be underrated in its effect. The chief remaining 

 factors of decrease are fusion of sporangia originally distinct, and abortion. 

 The former of these has probably been overestimated hitherto in its 

 evolutionary effect : the assumption has been too generally made that 

 synangia are the result of fusion of sporangia originally separate. Each 

 such case must be considered on its merits, but with the full conscious- 

 ness that septation will produce results structurally similar to those of 

 fusion. Abortion has been altogether underestimated in dealing with 

 early Vascular Plants. In the Lycopods there is ample evidence of its 

 effect ; and it is to be remembered that where abortion is complete, 

 no vestige remains to show what has taken place (Chapter XIII.). 



The condition of any poly-sporangiate sporophyte, regarded from an 

 evolutionary aspect, may be held to be the resultant of such conflicting 

 factors of increase or decrease as those mentioned, which were operative 



