1907] 



CHRYSLER— POT AMOGETON A CEA E 



163 



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arc the two fused stipules, sheathing the stem; their main bundles are 

 barely visible at this magnification, but are situated at the angular 

 swellings. Between the stipules and the stem is an axillary bud. The 

 stem shows the lacunar cortex characteristic of aquatics, and a well- 

 developed central cylinder. Fig. ij shows certain of these features 



more clearly, and also makes visible the numerous cortical strands 

 which lie in the angles of the meshes. 



Since the course of the leaf traces furnishes a key to the structure 

 of the internodal stele, it is proposed to describe the course of the 

 bundles in a wxll-developcd species, P. natans. Stated briefly, the 

 seven or more leaf traces shown in fig. 11 fuse to form three, which 

 enter the central cylinder, pass down through one internode sepa- 

 rately at the periphery of the central cylinder (see DeBary 7, p. 270.), 

 then approach one another, pass down through a second internode in 

 the form of w^hat may be styled a "trio" of bundles, and at the third 

 node become indistinguishable on account of union with the bundles 

 of the central cylinder. Since the leaves are arranged alternately in 

 two vertical rows, there are two groups of leaf trace bundles in the 

 cross-section of any internode. This is illustrated in fig. 14, which 

 will be rendered clear by the following diagram (fig. i), in which 

 tiy T^y /j represent the traces of the next 

 higher leaf, /^, T^, t^ those of the second 

 higher leaf, while the remaining bundles are 

 those which are styled by DeBary cauline, 

 since they do not lead directly to any leaf. 

 Although the petiole contains seven or more 

 bundles, as shown in fig. 11, they enter the 

 central cylinder as three, since the lateral 



bundles on each side fuse with one another. On each side this fusion 

 bundle is joined also by the median bundle of the stipule of that side. 

 An anastomosing branch may be sent to the median leaf trace. The 

 bundles at the base of the petiole, like those of the internode, are 

 devoid of xylem, but a lacuna represents the absent vessels, and shows 

 that the bundle is collateral. As the bundle enters the cortex of the 

 stem, ringed vessels or tracheids make their appearance, and the 

 bundle becomes somewhat swollen and almost amphivasal, owing to 

 increase in the number of tracheids which come more and more nearly 



Fig. 1 



