124 VARIATIONS IN NUMBER OF SPORANGIA 



morphology of Vascular Plants ; while the regularity and constancy in 

 number and position of the sporangia in the Lycopods, Equiseta, and 

 Eusporangiate Ferns, in which interpolation does not occur, has been 

 underestimated. But these, on grounds of comparison, and of fossil history, 

 are among the most primitive of Vascular Plants. Thus interpolation of 

 new sporangia is to be recognised as an actual factor of increase in number 

 of sporangia, but it is not a general phenomenon, and there is reason to 





FIG. 66. 



Sorus. of Davallia gritfflhiana, Hk. Showing sporangia of different ages irregularly 



intermixed. X 100. 



think that it has been initiated as a secondary character, and in certain 

 groups only. 



(c) Continued apical growth of the parts bearing the sporangia is a 

 marked feature in most Vascular Plants : a concomitant of it, in the case 

 of axes, is a continued embryology, with the initiation of an indefinite 

 number of successive primordia of spore-producing parts. This is con- 

 spicuous in the axes of many Lycopods, and especially so in the Selago 

 group of Lycopodium, where it appears to be unlimited : in other species of 

 the genus the apical growth of the strobilus also exists, but is of shorter 

 duration (Fig. 67). Much the same is the case in other strobiloid types, 

 with varying duration of the apical growth. The apical growth of the axis 

 is apt to be less prominent where the appendages are large, as in the 



