170 MORPHOLOGY 



aposporous gametophytes in ferns arise usually from checked sporangia, 

 but sometimes directly from the leaf margin (usually a tooth), and occa- 

 sionally from other regions of the leaf. 



An interesting question connected with apogamy and apospory is the effect on 

 the number of chromosomes. A sporophyte has the double (diploid) number 

 (2*) of chromosomes because it has come from a fertilized egg. Therefore, does 

 an apogamous sporophyte have the reduced (haploid) number (x) ? A gameto- 

 phyte has the reduced number (x) because it has come from a spore produced 

 by the reduction divisions. Therefore, does an aposporous gametophyte have the 

 double number (2 5c) ? Recent investigations, both among ferns and seed plants, 

 indicate that both of these questions may be ansvi^ered in the affirmative. 



Conclusions. — The important features of this great group may be 

 summarized as follows: .There are represented all the pteridophyte 

 types of vascular cylinder, from the most primitive to the most advanced; 

 but the cylinders are peculiar among pteridophytes (Ophioglossales 

 excepted) in being phyllosiphonic. The leaves are the most highly 

 developed among pteridophytes (Ophioglossales excepted), being re- 

 duced in number and increased in size. The sporangia are multiplied 

 on the lower surface of the leaves; there is very little differentiation of 

 foliage leaves and sporophylls; and there is no organization of strobili. 

 The conspicuous features are the development of the leptosporangiate 

 habit, found in no other group of vascular plants (water ferns excepted) ; 

 and along with this the appearance of a special kind of antheridium devel- 

 opment. There is a gradual diminution of spore output per sporangium, 

 and a corresponding diminution of sperm output; also a persistent re- 

 tention of homospory. The gametophyte is a thallose, aerial structure. 

 The line as a whole appears to be highly specialized, the only group con- 

 taining the possibihties of the higher plants being the INIarattiaceae, 

 probably the oldest family. 



(b) Hydropteridineae 



General character. — The water ferns are probably an aquatic 

 branch from the true ferns that has developed heterospory. They 

 are leptosporangiates, and hence are doubtless derived from the only 

 other known group of leptosporangiates. They comprise two distinct 

 families that seem to be of separate origin. It is evident, therefore, 

 that heterospory has arisen independently in almost every group of 

 pteridophytes. 



