GENERAL MORPHOLOGY 374 
leaf-sheaths, and capable of spore-production 
by a terminal strobilus. The branching, 
however complex, may be held as accessory, 
as also the formation of roots so closely 
associated with the branches. The funda- 
mental idea of the plant is thus carried 
back to the first shoot which originates 
with the embryogeny. It may be held that 
from this, by successive accessory branch- 
ings, the complex shoot-system arose, while 
the spore-production was deferred to the 
later branchings: it 1s on these that the 
fructification ultimately appears in the living 
species, while the primary axis and earlier 
branchings are normally sterile. 
The strobilus itself consists of a con- 
tinuation of the axis which bears it, and 
upon this the sporangiophores are disposed, 
but often with less regularity than rules in 
the case of the leaf-sheaths. The whole 
strobilus is normally occupied by the spor- 
angiophores, without any intervening bracts 
(Fig. 195 A). The sporangiophore itself 
consists of a central stalk supporting a 
polygonal distal end: from the margin of 
this the sporangia hang in variable number, 
forming a series surrounding the stalk (Fig. 
195 B). The spores are all of one type 
(Isosporous). At the base of the strobilus 
a ring-like structure is found—the annulus 
—which is like a reduced leaf-sheath, and it 
has usually been held to show a transitional 
stage between the vegetative leaf-sheaths 
and the first whorl of the sporangiophores, 
these being recognised as equivalent parts. 
Reasons will be advanced below for not 
accepting this apparently simple view. The 
strobilus of Eguzsetum is liable to variations 
of development, which have their importance 
in relation to certain fossil forms. The most 
notable of these is proliferation, the apex of 
the strobilus being continued as a vegetative 
shoot: the effect is thus gained of a fertile 
zone bearing sporangiophores, threaded upon 
an axis, or of a succession of such zones, 
Fic. 194. 
Equisetum sylvaticuin, L., forma poly- 
stachya, Milde. Plant with 18 secondary 
strobili, in three whorls of branches which 
are normally sterile. Natural size. (After 
Luerssen.) 
