227 



stem , in P. undulatus the stem is compressed with a more 

 or less deep ridge on each of the two broad sides [Fig, 1, B 

 and Fig. 3, ^] ; this characteristic form of the stem is also 

 found in P. crispus [Fig. 1, C] , but not in any other Danish 

 form of Fotamogeton, and as far as I know, not in any other 

 Potamogeton. 



A comparing examination of the structure of the stems 

 and the leaves further proves, that the other parent must 

 be P. praelongus and not P. perfoliatus. In P. undulatus 

 numerous vascular bundles are found in the bark [Fig. 1, 

 B, /A-], and these bundles are not to be found in P. crispus 



Fig. 2. 



Transverse section of the axial cylinder of: 



A. Potamogeton f erf oliatus. D. P. praelongus. K. P.cris\yus; 



the white parts: vascular bundles; the hatched parts: pith; 



Ig. lacunae in the vascular bundles, ("/i). 



[Fig. 1,C]; in P. perfoliatus are only found a few feeble 



bundles, or none at all; P. praelongus on the contrary has 



numerous bundles in the bark [Fig. 1, A, Ih]. As to the 



structure of the axial cylinder the proportion will most 



easily be seen by a comparison with Fig. 18iC [Fig. 2, A'] 



(P. crispus), Fig. 52 B and C [Fig. 3 B and C] (P. undulatus) 



and Fig. 18 A [Fig. 2, A'\ (P. perfoliatus) ; it appears from 



this, that the structure of the axial cylinder in P. undulatus 



differs from P. crispus and approaches to P. praelongus, 



and sometimes perfectly resembles the latter in this point 



(comp. 3, C with 2, D) , while there is no resemblance at all 



with P. perfoliatus. 



15* 



