6 J. O. HAGSTROM, CRITICAL RESEARCHES ON THE POTAMOGETONS. 



scheme. A cross-section of the stele exhibits three separate parts or areas: a median 

 area and two lateral ones. A fusion of bundles can take place with the median 

 bundles as well as with the lateral ones. In the median area there are four bundles 

 originally, of which three form a group belonging to the internode next above, and 

 the fourth, the opposite bundle, belongs to the leaf next above. The first and 

 commonest mode of fusion consists in the three bundles first mentioned, or the so- 

 called trio (Chrysler), uniting with each other into a compound bundle with three 

 phloem parts, or into a bundle with a single phloem portion, a trio-bundle, in both 

 cases with a common xylem cavity. The third stage finally comes about when the 

 trio-bundle unite with the opposite bundle forming a compound median bundle with 

 two phloem parts but a common xylem cavity in the centre. This central canal is 

 very constant and characteristic to many species, for inst. P. gramineus. In some 

 species the two median bundles run separately at the very base of the stem, unite 

 a couple of internodes above the bottom thus forming the typical characteristic up 

 to the internodes next below the primary spike, where they again separate running 

 lastly as a trio-bundle with its opposite bundle. In order to learn the characteristic 

 fusion you therefore should take the crosscut of an internode situated rather far 

 from the primary spike or at least a couple of internodes beneath it. 



The lateral vascular bundles also frequently coalesce with the single bundle 

 belonging to the leaf, the foliar bundle. Or the cauline bundles quite fail for want 

 of space at the side of the foliar one, or they are not required for the life of the 

 plant. In these cases of fusion or development there exist only two lateral xylem 

 canals, one on either side. The last stage of fusion enters, when the lateral foliar 

 bundles unite with the median foliar bundles thus forming a single compound bundle 

 with four phloem parts and one xylem cavity running in the centre. The crosscut- 

 form of such a stele becomes circular or nearly so. 



As to the anatomical conditions of the stele in general, the disposition of the 

 fibro-vascular bundles and the crosscut-form of it we thus observe the following six stages: 



A. Lateral bundles free, 2 — 4 (5) in number on each side. 



1. The prototypic diagram: four median bundles. Fig. 1, A—B. 



2. The trio-type diagram: a compound trio-bundle and an opposite bundle, 

 thus two median xylem canals, a frequently occurring case. Fig. 1, C. 



3. The eight-bundled diagram: usually six lateral and always two median 

 bundles, each with a single phloem part looking at the endodermis. Fig. 1, D. 



B. Lateral bundles only two, one on each side viz. the foliar bundle. 



4. The four-bundled diagram: two median and two lateral bundles, the first 

 mentioned often separated by a partition wall of mechanic cells. Fig. 1, E. 



5. The oblong diagram has two lateral bundles and one compound median 

 bundle, consequently three xylem canals often separated by a mechanic tissue. 

 Common to a plurality of species. Fig. 1, F — H. 



6. The circular diagram characterized by a large compound bundle with four 

 phloem parts occupying nearly the whole tube. In many species with a filiform stem 

 like that of P. panormitanus, trichoides and so on. Fig 1, K — L. 



