RESULTS OF PAL^ONTOLOGICAL RESEARCH 69 



With the chronologically later ferns the venation 

 becomes feathered, i.e. there are continuing main veins 

 with lateral branching minor ones (Fig. 21). 



In the rich Carboniferous flora there then appear 

 fronds with reticulated venation in which the lateral 

 branch veins are united together by numerous short 



b 



FIG. 21. a, PECOPTERIS creopteridia, Saar 

 district, b, c, d. Fronds (parts of) of various 

 ferns, showing spore heaps on the under 

 side. (After Gotfian.) 



FIG. 22. a, ALETHOPTERIS Serli, 

 Saar district, b shows reticulate 

 venation. (After Gothan.) 



connective ones (anastomosing) (Fig. 22, b). From 

 the purely comparative point of view the reticulate 

 venation is decidedly an advance over the feathered 

 venation, and this, in its turn, over the fan venation. 1 



1 Haberlandt : Physiologische Pftanzenanatomie, Leipzig, 1909, p. 348. 

 Haberlandt describes two chief types of venation in leaves. The first, in 

 which the veins proceed separately (without anastomosing), appears as a 

 rule in such leaves as never require much water or nutriment on account 

 of their smallness or trifling transpiration, and show assimilative activity. 

 The second type (reticulated) appears in leaves of the opposite character. 

 * Thus (by this venation) the leaf area, with the least possible length of 

 veins, becomes uniformly and by the shortest way supplied with water 

 and nutritive salts ' (p. 349). 



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