316 



GENERAL BOTANY 



Asexual reproduction. The erect branches terminate in the 

 reproductive strobili, or cones, formed of modified leaves, which 



bear the sporangia and 

 spores. These very highly 

 modified leaves are called 

 sporop hylls, w hich me an s 

 leaves that bear spores. 

 The sporophylls and spo- 

 rangia of the lycopods and 

 selaginellas are very simi- 

 lar except that in Selagi- 

 nella there are two kinds 

 of sporangia and two kinds 

 of spores. The selaginel- 

 las are therefore said to 

 be heterosporous, or with 

 different spores, to dis- 

 tinguish them from the 

 common ferns, equiseta, 

 and lycopods, which are 

 homosporous, that is, with 

 one kind of spores. It 

 should be stated also that 

 some of the true ferns 

 now living are heterospo- 

 rous, and that many of the 

 fossil Pteridophyta of the 

 coal period bore two kinds 



of spores. Since the heter- 

 osporous condition is of 

 the greatest importance 

 for an adequate under- 

 standing of the life his- 

 tory of seed plants, the 

 following description of the sporangia and spores of Selaginella 

 is given to explain the nature of heterospory in a common tropi- 

 cal species of the Lycopodiales. 



FIG. 181. The habit of a lycopod 

 (Lycopodium) 



A, plant with horizontal rhizome and erect, leafy 

 spore-bearing stem ; str, strobili ; B, spores ; 

 C, sporophyll and kidney -shaped sporangium. 

 From Bergen and Caldwell's "Practical Botany" 



