GAMETOPHYTES 437 
aérial shoots in the epidermis of which are stomata through 
which carbon dioxide and oxygen reach the cortex, 
The fertile branch commonly appears first in the spring, and 
in some common forms of Equisetum bears no side branches, 
thus having only whorls of scale-like leaves at the nodes. At the 
apex of the fertile branch is borne the strobilus (plural strobili) 
which is so named because of its resemblance to a cone such as 
occurs in Pines (A, Fig. 390). The strobilus consists of a central 
axis (the prolongation of the axis of the branch) to which are 
attached the stalked shield-shaped structures or sporangiophores, 
so named because they bear sporangia (C, Fig. 390). Some re- 
gard the sporangiophores as modified leaves and, therefore, call 
them sporophylls, which means spore-bearing leaves, but until 
their relation to leaves is definitely determined, sporangiophore 
is the safer term. Under the shield-shaped top of the sporan- 
giophores are borne the sporangia, ranging from five to ten in 
number on each sporangiophore. The spores are provided with 
ribbon-like appendages, called elaters, which become entangled 
and thus cause the spores to fallin clumps. The spores, although 
alike in size, are physiologically different, for some of them pro- 
duce only male while others produce only female gametophytes. 
In some species of Equisetum the fertile branch dies after the 
spores are shed, but in others the strobilus falls off and the branch 
continues to elongate, becomes green, and makes food during the 
remainder of the growing season. 
There are two notable features presented by the sporophytes 
of the Equisetums. One is the differentiation of the aérial por- 
tion of the stem into sterile and fertile shoots. The second is the 
aggregation of sporogenous tissue into a strobilus. The sterile 
branch is a means by which sporangia can be elevated, so that the 
spores are in a good position to be scattered. The strobilus is 
supposed to be the forerunner of the flower, which likewise is a 
structure consisting essentially of aggregates of sporogenous tis- 
sue, for the pollen grains are spores, and also in the ovules there 
are spores developed. 
Gametophytes. —In the Equisetums the gametophytes are 
much more reduced than in the True Ferns (Fig. 391). They are 
so small that one needs 4 lens to identify them, Unless conditions 
are very favorable, they are not able to survive out of doors, and 
consequently the Equisetums are propagated principally vegeta- 
