DEVELOPMENT OF PLANTS 



323 



plant is one of the significant departures that appears in the evo- 

 lution of the plant. This distribution of labor that is seen in the 

 setting aside of certain groups of leaves for spore production 

 will appear in all the succeeding groups ; and the arrangement and 

 structure of the sporophylls will steadily become more and more 

 complex until a point is reached where they are popularly called 

 a flower, although this term can be just as correctly applied to 

 the strobilus appearing in Equisetum and succeeding groups. 

 The sporophylls of Equisetum originate at the nodes, as in the 

 case of the scale leaves, but as they enlarge the apical portion 

 spreads out like a shield. The internodes do not elongate and 

 separate the sporophylls to any considerable extent, consequently 

 the shields become six-sided through contact with the adjacent 

 sporophylls. 



Fig. 234. 



Fig. 235. 



Fig. 234. Sporophylls and spores of Equisetum: 2, sporophylls viewed 

 from outer and inner side, showing form and attachment of sporangia and 

 the central stalk attaching the sporophyll to the strobihis. 3, spores with 

 elaters expanded in a and partially coiled in b. 



Fig. 235. Female gametophyte of Equisetum bearing several archegonia 

 and leaf-like lobes. At right male gamete. — After Sadebeck. 



{b) Character of Sporangia and Spores. — The sporangia ap- 

 pear as rather elongated sacs on the under side of the shields 

 (Fig. 234, 2) and at maturity open by a longitudinal cleft. The 

 structure of the spores is of especial interest. The outer wall 

 of these spores is thickened in spiral bands, and owing to the 

 dissolution of the thin portion of the wall separating these bands, 

 the outer coat uncoils and appears as four bands, with spoon-like 



