108 



ELEMENTARY STUDIES IN BOTANY 



of them certain branches of the leaf bear sporangia and differ 

 very much in appearance from the foliage branches (Fig. 



88). But in no case 

 are sporophylls grouped 

 together to form strobili. 



62. Heterospory. - 

 In most of the Pterido- 

 phytes, all the spores 

 produced by the spo- 

 rangia are alike, both in 

 appearance and in the 

 gametophytes they pro- 

 duce. This condition is 

 called homospory (" simi- 

 lar spores "), and such 

 plants are homosporous. 

 In some Pterido- 

 phytes, however, notably 

 one kind of club-moss 

 (Selaginella, Fig. 71), the 

 spores are not all alike 

 (Fig. 89). They differ 

 very much in size, the 

 large ones being called 



FIG. 89. Selaginella (a club-moss) : A, a spo- t megdSpOreS (" large 



rophyll (microsporophyll) bearing the spo- 

 rangium (microsporangium) that produces 

 small spores (microspores) ; B, microspores 

 (lowest one separate, upper ones clinging 

 together) ; C, a sporophyll (megasporophyll) 

 bearing the sporangium (megasporangium) 

 that produces large spores (megaspores) ; 

 D, two megaspores, drawn to the same scale 

 as the microspores (B). 



spores ") and the small 

 ones microspores ("small 

 spores"). Not only do 

 they differ in size, but 

 they differ also in the 

 gametophytes they pro- 

 duce, the megaspores producing female gametophytes (that 

 is, gametophytes that bear only archegonia), and the 

 microspores producing male gametophytes (that is, gameto- 

 phytes that bear only antheridia). This condition is called 



