GENERAL MORPHOLOGY 



371 



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 is 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 Eqitisetum 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, 



FIG. 194. 



Ei/i<!sctnin syk'ii/icuii/, L., forum poly- 

 si achy a, Milde. Plant with 18 secondary 



stroliili. in three whorls of br.'uic lies which 

 are normally sterile. Natural si?e. (After 



