STRUCTURE 



435 



stem. Al each node is a transverse })arliliun separating the 

 internodal spaces from one another. Around the central 

 cavity, and corresponding with the longitudinal ribs with 

 which the stem is marked, is a series of smaller air-cavities 

 ((•'), arranged in a circle, and alternating with these, between 



Fio. 112. — A, portion of aerial shoot of Equisetum, showing a node 

 (aJ) from which arise a leaf-sheath (/. sA) and a whorl of secondary 

 shoots (5/t). (Nat. size. ) 



B, transverse section of aerial shoot, showing central (t') and peri- 

 pheral (('-) air-cavities, and ring of vascular bundles with smaller air- 

 cavities (^). ( X 2. ) 



c, .1 single sporophyll {sp. ph) with stalk (i/) and sporangia {spg). 

 (x 10.) 



D, a single spore showing coiled elater {el). 



K, the same, with elater {el) expanded. 



(A-c, after Goebel ; D and E, after Le Maout and Decaisne. ) 



them and the central cavity, are the vascular bundles i^v. b\ 

 each with a small air-cavity (r*) in its inner or central 

 portion. 



The microscopic structure of the plant agrees in essential 

 respects with that of the fern, though differing in many 

 details to which no further reference need be made here. 



F F 2 



