312 



GENERAL BOTANY 



FIG. 179. Equisetum arvense 



A, plant in early spring, with aerial branch (sb) 

 bearing strobili (c) and a leafy branch (fb) ; B, a 

 green summer shoot; C, sporangiophore with 

 stalk (st) and sporangia (sp) ; D, spore with ex- 



the fact that all higher green 

 plants had similar stages and a 

 like sequence of stages in their 

 life histories. In the brief discus- 

 sion which follows, the life histo- 

 ries of the equiseta and lycopods 

 will be compared with that of 

 the fern, in order to extend the 

 conceptions that have been gained 

 with regard to the life history of 

 ferns and of the fern allies. Sela- 

 ginella, with its two kinds of 

 spores, will be taken as a rep- 

 resentative of the Lycopodiales, 

 in order to introduce the stu- 

 dent to the problems connected 

 with the origin of the seed and 

 of the seed habit in the highest 

 green plants. 



EQUISETALES, OR HORSETAILS 

 SPOROPHYTE 



Habit. The sporophyte plants 

 of the equiseta differ in impor- 

 tant particulars from those 

 of the ferns on account 

 of their xerophytic form 

 and structure (Fig. 179). 

 Some of the modern equi- 

 seta, like a few of the ferns, 

 inhabit wet marshes and 



ponds, but they all retain 



panded elaters ; E, spore with contracted elaters J 



the xerophytic structure 



and appearance which is derived from their treelike ancestors 

 of the coal period. The xerophytic structures of the equiseta 

 are therefore genetic and not adaptive characteristics. 



