1907] CHRYSLER— POTAMOGETONACEAE lyy 



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account of the earlier work. Campbell (3) has studied N. flexilis 

 by the aid of modern technique, and Rendle (22) has recently 

 revised the genus from the systematic standpoint. The last two men- 

 tioned authors summarize our knowledge of the morphology, so we 

 may at once proceed to a discussion of the features which throw hght 

 on the affinities of the genus. My ow^n observations have been con- 

 fined to two species^ N, -flexilis and N, marina, obtained from several 

 different localities with a view to confirming earlier accounts- The 

 structure of the stem resembles that found in Zannichellia^ and is figured. 

 for typical species by Rendle; a cortex containing a circular row of 

 lacunae incloses a slender stele which consists of thin-walled elongated 

 cells surrounding a central lacuna. The endodermis is usually well 

 marked. A single leaf trace enters the stele directly from each leaf. 

 In N, 'flexilis a few ringed or spiral tracheids are to be seen at the 

 nodes, especially the younger ones, also in the pedicel of the flower, 

 and, according to Campbell, in the young leaves as well. In N. 

 marina, however, I am unable to find any trace of tracheary tissue, 

 although this species is a stouter form than A^. flexilis. The feature 

 to which most significance has been attached is the simple flower, 



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consisting either of a single ovule or single anther, inclosed in a 

 ^'spathe." In early stages the micro- and megasporangiate struc- 

 tures are scarcely distinguishable, and Campbell compares them with 



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the sporangia of Azolla. This author argues for the derivation of 

 the monocotyledons directly from pteridophytes, especially the Isoe- 

 taceae, through simply organized aquatics such as Naias or Zanni- 

 chellia. According to this view the simplicity exhibited by the 

 vegetative and reproductive structures is primitive. In marked con- 

 trast is the opinion of Magnus: "Bei Naias scheint mir vicles dafur 

 zu sprechcn, das wir es mit reducirter Einfachheit zu thun haben" 

 (18, p. 222). It will be well to examine the evidence in some detail. 



(i) On the view that Naias represents a reduced form derived from 

 a land plant, the occurrence of tracheids at the nodes, in the pedicels 

 of the flowers, and in the young leaves, receives ready explanation, for 

 these regions are just the ones in which ancestral features are apt to 

 persist. On the other hand, there seems to be no reason why a sub- 

 mersed aquatic which is primitively an aquatic should have any tra- 

 cheids whatever. Under this head the evidence seems conclusive. 



