266 



THE SPORE-PRODUCING ORGANS 



[CH. 



smaller output it becomes a mechanism highly specialised for ejection of the 

 spores. All of these features indicate evolutionary progression with decrease 



Fig. 261. A, section of sporangium of Gleichenia JJabellaia, showmg 

 ovei 60 spore-mother-cells in section. ( x 100.) B, spore-mother- 

 cells of the same plant, older, and separated from one another in 

 the tapetal plasmodium. ( x 165.) C, spore-mother-cells and 

 tapetal plasmodium of Gleichetiia dichototna; only -20 are seen in 

 section. ( x 100.) D, cells separated in tapetal plasmodium. 

 (X165.) 



in size and simplification of structure of the sporangium, and a fall in the 

 individual output of spores, but a perfecting of the mechanism of their 

 distribution. This is seen as we pass from the relatively primitive and 

 geologically early Ferns to those which Palaeontology and comparison both 

 indicate as later in evolution. 



Notwithstanding the progressive fall in the spore-production of the 

 individual sporangium, the output per sorus may remain approximately the 

 same. This is shown by comparison of Ferns systematically remote from 

 one another. For instance the estimated production per sorus for Marattia 

 fraxinea is 45,000, for Polypodiiim aureiim 57,000, iox Angiopteris evecta 14,500, 

 for HyinenophyllMU dilatatum 1 1,500, for Alsophila excelsa 3200, for Gleichenia 

 flabellata 3000. The similarity in output in such pairs of cases may be set 

 down merely to similarity of the underlying nutritive mechanism. Such 

 comparisons become more interesting when the plants compared are of 

 nearer affinity, as in the Hymenophyllaceae, in which the sorus has a uniform 

 type of construction though the size and number of the sporangia, and the 

 length of the receptacle, are variable. The results of comparison are given 

 in the subjoined table: 



