368 



PRIMARV ARRANGEMENT OF TISSUES. 



outside of the wide intercellular passage of the sympodial bundle, while two some- 

 what smaller groups stand symmetrically right and left of the centre of its inner side. 

 In the remaining bundles the intercellular passage or vascular group is bordered on 

 the outside by an arched group of sieve-tubes. 



In the other Potamogetons investigated (lucens, gramineus, densus, crispus, pec- 

 tinatus, and pusillus), in Zanichellia, Althenia, Cymodocea, and Zostera, the tracheae 

 in the node are persistent, while those in the internode are all transitory. To every 

 bundle an intercellular passage corresponds, which is surrounded on the outside by 

 phloem. Where several bundles traverse the internode, they approach each other 

 closely in a manner similar to that described for P. natans ; in the case of the two 

 leaf-trace bundles in the internodes of P. lucens and gramineus this goes so far, that 



Ir^Tv 



'V^mmT' 



FJG. 170 (145). — Potamogeton natans. Axial mass of the internode, containing the vascular bundles ; cross-section. 

 It unilaterally thickened endodermis, containing starch ; outside the latter lacunose cortical parenchyma, with 

 abundant starch ; / air-cavities. Explanation of the numerals at p. 272. The delicate groups of tissue of the 

 numbered circles are the phloem ; the wide meshes in the latter are the sieve-tubes of the bundles ; the circles in 

 which the numerals stand are the xylem portions, usually converted into cavities. Between the bundles is parenchyma 

 containing starch, and sclerenchymatous fibres with a narrow lumen appearing as a darker point. 



their intercellular passages, which are turned towards each other, are only separated 

 by one layer of cells, or in most cases are united to form a single passage. 



The single axial sympodial bundle, which (without cauline bundles) traverses 

 the internode in the upright stem of P. pectinatus and pusillus (Fig. 171), has, in the 

 manner of a concentric bundle, a central intercellular passage replacing the xylem- 

 group, and this is completely surrounded by a relatively bulky phloem, containing 

 large sieve-tubes, and externally limited by the endodermal sheath, which eventually 

 becomes sclerotic. 



The bundles in the stems of Zanichellia and Althenia behave quite similarly to 

 the forms last mentioned, only with the difference that the phloem is very delicate 

 and slightly developed, and consists of elongated cells with a few indistinct sieve-tubes. 

 Elodea and Hydrilla, of which we shall speak later on, are also connected with these 

 cases. 



