IMPERFECT AND RUDIMENTARY BUNDLES. 



369 



The vascular groups of Cymodocea sequorea ' and Zostera behave, both in the 

 nodes and in the internodes, like those of the Potamogetons. The intercellular canal 

 derived from the xylem lies in the small peripheral bundles on the inner side ; outside 

 this is a radially elongated phloem-group containing two or three large sieve-tubes ; in 

 the thicker axial bundle the intercellular space occupies the middle, and in the case of 

 Cymodocea has at its periphery four sieve-groups placed cross-wise in the transverse 

 section ; in Zostera it is completely surrounded by a broad phloem, as in Potamo- 

 geton pectinatus. 



The small stems of the Hydrilleae 

 and of Aldrovanda vesiculosa consti- 

 tute transitional forms between the 

 bundles which become incomplete and 

 those which remain rudimentary. 



Elodea canadensis and Hydrilla 

 verticillata have an axial bundle of es- 

 sentially similar structure to that of Zani- 

 chellia. The one or two axial tracheae 

 present in the young rudiment of the 

 stem, which send off a branch into each 

 leaf, are incomplete from their first origin ; 

 their walls are only thickened with seg- 

 ments of rings, and disappear every- 

 where — even in the node — on the com- 

 mencement of active extension^- Ac- 

 cording to Caspary, Aldrovanda shows 

 an axial bundle of 8-9 annular tracheae, 

 which, together with their branches 

 going to the leaves, are persistent in 

 the nodes, but disappear in the inter- 

 nodes during extension, and are here re- 

 placed by a passage surrounded by 

 delicate-walled elongated elements, 

 which have not been more minutely 

 investigated. Of bundles which remain 

 rudimentary, those in the small stem 

 of Ceratophyllum and Najas are im- 

 mediately related to those just described. According to Sanio the former are 

 bundles which are at all times destitute of vessels, and consist of a mantle of narrow 

 sieve-tubes and elongated cells, between which lie several small intercellular pas- 

 sages, each produced by the absorption of a row of cells'; and inside this 

 mantle are some layers of parenchyma, which surround an axial passage derived 

 from the absorption of a multiseriate strand of cells. In the case of Najas the 

 mature structure is similar to that of Elodea; sieve-tubes have not been described, and 

 are doubtful; the axial canal arises from the absorption of a row of meristematic cells. 



FTG. 171.— Potamogeton pectinatus (80). Cross-section through 

 an internode of the upright stem, ^intercellular passage replacing 

 the xylem which has disappeared ; u unilaterally thickened endo- 

 dermis ; between w and g the phloem, with wide sieve-tubes. ' 

 Between u and the epidermis e is the lacunose cortex ; b bundles 

 of sclerenchymatous fibres ; a a similar bundle, with a small group 

 of sieve-tubes in the middle. 



' Compare Bornet, I.e. 



Caspary, Sanio, I.e. (p. 278). 

 Bb 



Compaie Frank, Beitr. p. 143, 



