338 PRIMARY ARRANGEMENT OF TISSUES. 



no evident sieve-tubes ; on its outer border are thick-walled cells, which latter in the 

 terrestrial species assume the characteristics of tough fibrous cells. On the border of 

 the phloem and xylem there lies in most species in the middle of the bundle a single 

 Intercellular canal, while in I. Engelmanni there are usually three of them ; their origin 

 is not clear \ The radial walls of the layer of cells bordering these canals have in 

 I. Engelmanni (Russow) and I. Durieui the characteristics of the radial walls of the 

 endodermis. No such structure is present at the periphery of the bundle. ■ 



In connection with Isoetes, Phylloglossum may be mentioned, as the short 

 description by Mettenius ' shows at least this one point of agreement, that its vascular 

 bundles only contain a few delicate tracheides, with annular thickenings, or spiral 

 fibres which can be unwound. The phloem is at any rate very inconspicuous ; 

 according to Mettenius it is even in many cases wholly absent. 



Sect. 103. The form of collateral bundles above designated as the double or 

 bicollateral is distinguished from the simple collateral form by having two groups of 

 phloem, one being situated as in the latter on the outside of the xylem, and a second 

 on its inner side. In all other respects they agree with the simple form.^. 



As the type of this form of bundle are to be mentioned in the first instance all 

 leaf-trace bundles of the Cucurbitacese, and in fact of all species investigated *- Both 

 groups of phloem have the typical structure described (p. 324, i), and are especially 

 remarkable for the size of their sieve-tubes (chap. V). They are frequently connected 

 by means of a narrow band, fringing the lateral edge of the bundle, and containing 

 some sieve-tubes, so that in these cases the bundle, strictly speaking, belongs to the 

 concentric type. The xylem is constructed altogether-on the collateral type ; on the 

 inside are narrow annular and spiral vessels ; towards the outside are reticulated vessels 

 becoming gradually wider, and finally very large pitted vessels with short articulations. 

 The latter are surrounded by broad layers of cells, some of which are elongated, with 

 thick pitted walls, while others are short elements with undulated surfaces fitting into 

 one another, and round-meshed reticular thickenings on their walls. It remains to 

 be investigated whether, or how far these elements should be called tracheides. 



A bicollateral structure is presented by the leaf-trace bundles of many Dicoty- 

 ledons, which belong to the ring : Melastomacese ^ Cichoriacese, Solanacese, Ascle- 

 piadeae, and Apocynese ; Strychnos, and Daphne. In many of these the inner phloem 

 IS so widely separated from the rest of the bundle, that it may be regarded as a 

 distinct strand of sieve-tubes ; in other cases distinct strands of sieve-tubes occur side 

 by side with the inner groups of phloem of bicollateral bundles, e.g. Cichoriacese, Sola- 

 num tuberosurh, and dulcamara ; comp. p. 2 3 r .' Of the Myrtacese already mentioned at 

 p. 231, Eucalyptus globulus decidedly belongs to this series. All the investigated 

 species of Eucalyptus, Metrosideros, Callistemon, Melaleuca, and Myrtus, have on 

 the inner side of the primary bundles a group of tissue consisting of delicate narrow 

 elements, and, according to what has been found in the case of Eucalyptus globulus, 

 It IS very probable that these groups have the same nature as those in the latter 



» Compare A. Braun, Isoetes-Arten d. Insel Sardinien, Monatsbericht. d. Berlin. Acad. 1863. 



* Botan. Zeitg. 1867, p. 99. 



= [Petersen, Ueber das Auftreten bicollateraler Gefassbiindel in verschiedenen Pflanzenfamilien. 

 Engler, Bot. Jahrb. 1882, p. 359.] 



* Compare Dippel, Mikroskop, p. 225 ; Bryonia. 5 Vochting, I.e. 



