GAMETOPHYTES 437 



of which are stomata through which carbon dioxide and oxygen 

 reach the cortex. 



Usually the fertile shoots are first to appear above, ground in 

 the spring. In some species of Equisetum the fertile shoots are 

 simple and in some species they are branched. 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 fall in clumps. The spores, although 

 aUke 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 aerial 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 a 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- 



