182 



BOTANY. 



membered that sterile strands of tissue are found in some of 

 the liverworts in the form of elaters. 



298. Microspores and macrospores. — The spores of isoetes are 

 of two kinds, small ones (microspores) and large ones (macro- 

 spores). When one kind of spore is borne in a sporangium 

 usually all in that sporangium are of the same kind, so that certain 

 sporangia bear microspores, and others bear macrospores. But 



Fig. 156. 

 Base of leaf of isoetes, 

 showing sporangium with 

 macrospores. (Isoetes en- 

 gelmannii). 



Fig- 157- 

 Section of plant of Isoetes engelmanii, showing 

 cup-shaped stem, and longitudinal sections of the 

 sporangia in the thickened bases of the leaves. 



it is not uncommon to find both kinds in the same sporangium. 

 When a sporangium bears only microspores the number is much 

 greater than when one bears only macrospores. 



For a discussion of the club mosses (lycopodium and selagi- 

 nella) and for a comparison of the ferns and fern-like plants, 

 see the author's larger " Elementary Botany," Chapters XXVIII 

 and XXX. 



