SPORE-PRODUCING MEMBERS 379 
Each of the sporangiophores, from which the sporangia thus depend, 
is traversed from the stalk upwards by a vascular strand, which branches 
in the enlarged head, and each branch terminates immediately below the 
base of one sporangium. The sporangiophores are in close juxtaposition 
while young, and thus the sporangia are effectively protected. At the base 
of the strobilus lies the annulus, which completes the investment of the 
lowermost series of sporangiophores: it has as a rule no vascular supply 
(Fig. 209). Goebel has pointed out the 
protective biological use of the annulus ;1 
also that at the apex the highest spor- 
angiophores may be imperfectly developed 
and concrescent, thus forming a terminal 
cap: the protection of the young sporangia 
is thus very complete. 
The number of sporangiophores in the 
Equisetum-strobilus is not strictly defined, 
while the number of sporangia on each 
sporangiophore is also variable: it is usually 
larger in Zguzsetum than in the Cala- 
marians: this raises the question of evidence 
of variability of number of sporangia. There 
is in Lgutsetum no structural evidence of 
the septation of sporangia such as might 
lead to their increase in number, nor is 
there any interpolation of later sporangia 
between those first formed. In some of 
the larger cones, such as £. maximum, 
branched sporangiophores are commonly 
found, which appear to indicate a possible 
increase in their number by fission: the Righben: 
irregularity of their number and arrange- — goyisetum arvense, L. A, section tra- 
: e versing a sporangium in median longitudinal 
ment in these large cones would seem to flane: the celle marked (x), like those in 
ql ‘ ‘ Fig. 206, are traced in origin from segmen- 
support this (compare Fig. 195.) Excepting tation of superficial cells subsequent to the 
for such indications there is no evidence eee #9 Hlulee Boor 
among living species of methods of increase 
in number of sporangia. Even the apical growth of the strobilus itself is, 
as a rule, strictly limited. Of reduction in number of sporangia there is 
as little direct evidence, but it is to be remembered that complete abortion 
leaves no trace of what has occurred (see Chapter X.). On grounds to be 
mentioned below it would seem probable that such complete abortion of 
sporangiophores has figured in the evolution of the Equisetales, contributing 
to the origin of the initial vegetative system of the individual plant. 
The structure of the mature sporangiophore and of the sporangia in 
the Calamarians is so similar to that of Lguzsefum that, taken together 
; 1 Organography, ii., p. 500. 
