644 GENERAL COMPARISON OF THE FILICALES 



of the sporangium, and in the thickness of its stalk and of the sporangial 

 wall, and a progressive change from the early segmentation characteristic 

 of the Eusporangiate type to that of the Leptosporangiate. 



As a consequence of such phyletic changes in the constitution of the 

 sorus, it is noteworthy how often the output of spores per sorus is 

 similar in Ferns which are systematically remote from one another : for 

 instance, Marattia fraxinea (45,000) and Polypodium aureiim (57,600); 

 Angiopteris evecta (14,500) and Hymenophyllum dilatatum (11,500); 

 Alsophila excelsa (3,200) and Gleichenia flabellata (3,000). These 

 examples show how a similar result may be obtained by various means, 

 a large number of small sporangia balancing a smaller number of large 

 ones. The similarity of output in such cases may be merely a conse- 

 quence of similarity in the powers of the underlying nutritive mechanism. 

 The real interest, however, arises when in nearer circles of affinity, with 

 varying size of sporangia, and of output per sporangium, the result per 

 sorus is kept approximately constant by converse variation of the two 

 factors. This is illustrated in the genus Gleichenia^ and in a less 

 precise way in Alsophila excelsa and Cyathea dealbata. But the best 

 demonstration of it is seen in the Hymenophyllaceae. undoubtedly a 

 very natural series, in which the sorus has a uniform type of con- 

 struction, though the size and number of the sporangia, and the length 

 of the receptacle are variable. In illustration of this, estimates have been 

 made with such accuracy as possible, with the results which are given 

 in the subjoined table : 



It thus appears that notwithstanding the great variations of sporangial 

 output, the result per sorus is approximately uniform for the cases 

 quoted from that very natural family of Ferns. This suggests a true 

 biological progression, and it probably does not stand alone, but 

 illustrates a principle which has been of wide application in the Fern 

 phylum. 



The production of numerous spores is a drain upon the resources of 

 the plant. That drain may be relieved within the sorus by the development 

 of a succession of sporangia, the demand being thus spread over an extended 

 period. In the Simplices the sporangia of a single sorus arise simultane- 

 ously : the physiological drain thus comes at one time : this method, which 

 is, physiologically speaking, a simple and probably a primitive one, is 



