172 BOTANICAL GAZETTE [September 



of the plants except the flowers, leads to the view that Ruppia is an 

 example of simplicity by reduction. 



POSIDOXIA 



Magnu 



oceanica (17). According to his description, the conducting tissue 

 of the central cylinder is flanked by a mass of mechanical fibers, and 

 the cortex contains numerous small bundles of fibers and also a few 

 conducting bundles. No account is given of the course of the leaf 

 traces, and material has not been obtainable so far for the present 

 research. 



ZOSTERA 



The widely distributed species Z. marina has been the subject of 

 frequent study. Eichler (10) described the sympodium of floral 

 axes, and Sauvageau (23) has given the best account of the vegetative 

 structures of the five species, referring to the older literature on the 

 subject. The creeping stem is monopodial, and bears the long ribbon- 

 shaped alternate leaves, while the ascending branches are sympodes 

 bearing the flattened spadices. In all these regions the main struc- 

 tural features are the same, namely, a wide, somewhat lacunar cortex 

 containing two vascular strands and inclosing a narrow central 

 cylinder. The structure of the last, however, differs considerably in 

 the various regions of the plant. . Fig. 2p, from the lower part of the 

 ascending shoot, shows the simplest condition, with a central lacuna 

 and four masses of phloem, therefore representing four fused bundles. 

 This condition is present also in the apical parts of the creeping stem, 

 and arises from the distichous arrangement of the leaves. The older 

 parts of the creeping stem show four additional bundles in the stele, 

 alternating with the first-formed bundles. Xylem fills the lacuna at 

 the nodal regions, but is nowhere well developed. From the sheath- 

 ing base of a leaf a median bundle enters the central cylinder directly; 

 while on each side two or more lateral traces fuse and join the cortical 

 bundle on its side, which at the same level sends a branch to the central 

 cylinder, as shown in the diagram, fig. 6. From the leaf also enter 

 a large number of small bundles of fibers which run down in the outer 

 region of the cortex; in the creeping stem these may be thin walled 



