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 
Sorus of Davallia griti/thiana, Hk. Showing spore 
intermixed. — X 100. 
gia of different ages irregularly 
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 Se/age 
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 
