78 PART I. THE MORPHOLOGY OF PLANTS. [ 16. 



phylls are generally small scaly leaves ; and in the Angiosperms 

 they differ widely in form from the foliage-leaves. In some of 

 the Algae too, as in Cladostephus (Sphacelariese), there are 

 definite sporophylls. 



The distribution of the sporangia among the sporophylls in 

 heterosporous plants is an important point. In the Hydropfcerideae 

 (Rhizocarpce), both the microsporangia and the macrosporangia 

 are borne by the same sporophyll ; but in all other heterosporous 

 plants they are borne by distinct sporophylls, which may be dis- 

 tinguished respectively as microsporophylls and macros porophylls. 

 In the Phanerogams the microsporophyll is termed a stamen ; the 

 macrosporophyll, a carpel] but there is no reason for confining 

 these terms to this group of plants. 



The term stamen is not, however, always strictly confined to microsporo- 

 phylls, but it is also applied to portions of the axis bearing microsporangia. 

 For instance, in Callitriche, Casuarina, Naias, etc. (see p. 71), the micro- 

 sporangia (pollen-sacs) are borne on a segment of the sporophore, which is 

 termed a stamen. 



In heterosporous plants, both kinds of sporophylls are gener- 

 ally present in one and the same flower: when, however, the 

 flower includes only microsporophylls, it is termed microsporophy l- 

 lanj or staminate ; and when it includes only macrosporophylls, it 

 is termed macrosporophyllary or carpellary. 



In some cases the sporangia are borne, not upon, but in close relation with, a 

 leaf, which is nevertheless regarded as a sporophyll. Thus, in Selaginella, the 

 sporangium is in the axil of the sporophyll. Again, the leaves which invest the 

 macrosporangia (ovules) of Polygonacese, Primuhcese, etc., are termed carpels, 

 though they do not actually bear the sporangia. 



The distribution of the sporangia on the sporophyll is various. 

 They may be borne exclusively on the under (dorsal) surface, as 

 in most Ferns, Equisetum and Gymnosperms (pollen-sacs) ; or 

 exclusively on the upper (ventral) surface, as in the Lycopodinse, 

 Marsileaceae, macrosporangia of Coniferae and of some Angiosperms 

 (e.g. Butomus) ; or on both surfaces, as in Osmunda ; or on the 

 lateral margins, as in Ophioglossum and the Hymenophyllacese, and 

 in many Angiosperms (e.g. Leguminosae, Violaceae, Liliaceae) ; or on 

 the apices of segments of the sporophyll, as in the Salviniaceee. 



The number of the sporangia borne by a sporophyll also varies 

 widely. In some cases there is only one, as in Selaginella, Lyco- 

 podium, Isoetes ; or two, as in most Coniferas ; or four, as in most 

 Angiosperms (microsporangia) ; or many, as in the Filicinse. 



