STRUCTURE OF COLLATERAL BUNDLES. 337 



wall of the primitive tracheae ; the latter are attached to the wall of the passage, and 

 if its expansion is considerable they may be laterally removed from one another, and 

 as the separation usually takes place before the elongation of the parts is complete, 

 they become simultaneously distorted in the longitudinal direction, and reduced to 

 thickening fibres adhering to the wall of the passage. This process often attacks 

 annular tracheae, the rings of which, in cases of great elongation, then become 

 shifted to a long distance from one another. The width attained by the passage is 

 various, sometimes equal to that of a moderate vessel, sometimes to the cross-section 

 of the whole persistent part. The cells actively engaged in its formation undergo, 

 on considerable expansion, divisions which are radial with reference to the passage, 

 and remain as a rule thin-walled. These passages contain air, with the exception of 

 some submerged plants to be mentioned below. 



All collateral bundles of the stem of Equiseta show a relatively wide passage at the 

 inner side of the xylem. The same phenomenon occurs very widely in the leafy stems 

 (halms) and leaves of numerous Monocotyledons, but not in their rhizomes : thus in the 

 stems of Hydrocharis, Butomus, Sagittaria, AHsma, Juncacese, Xyris, Cyperaceae, Acorus 

 Calamus, Leucojum, and Commelineae (Tradescantia albiflora, zebrina, Lyonii^). As the 

 names show, most but not all of these plants inhabit water or bogs. Further, the pheno- 

 menon does not occur in all the bundles of the same part ; e. g. the smaller bundles in 

 the leaf and scape of Acorus Calamus have no passage, while the larger bundles have a 

 very wide one ; and further, nearly related plants, agreeing in their mode of life (e. g. 

 Grasses, Cyperaceae, Commelineae), often show a different character with reference to the 

 width of the passage, and its presence or absence. Of Dicotyledons only some water- 

 plants belong to this series, namely, the water Ranunculi and Nelumbium, besides those 

 to be mentioned below. Both have a passage on the inner side of the larger, but not 

 of the smaller vascular bundles of the stem. 



In a number of water-plants the process described extends to the complete 

 destruction of the whole xylem. The latter originates at an early age in the form of 

 a few annular tracheides, or of a large number arranged in a bundle, all of which 

 become both separated from one another laterally, and torn longitudinally, as the part 

 elongates. The inconspicuous separate rings or fragments of rings remain 

 adhering to the wall of the passage, the phloem, which is usually strongly developed, 

 is alone persistent. As far as the investigations extend, these passages contain 

 water. The leaf-trace bundles of the internodes of Potamogeton natans and its 

 alhes belong to this series ; — in their cauline bundles and in the nodes the tracheae 

 are persistent — also the bundles of the leaf-stalk and flower-stalk of species of 

 Nymph sea and Nuphar, and of Brasenia peltata '■'. In many bundles of the plant last- 

 named a portion of the vessels are persistent; they thus belong to the former 

 category ; in their rhizomes no passages occur in the vascular bundles. — On allied 

 phenomena in the non-collateral bundles of other water-plants, comp. Sect. no. 



A formation of passages in some degree different from that described occurs in 

 the flower-stalks, leaf-stalks, and leaves of Aroideee, especially of those with unisexual 

 flowers, as Colocasia, Caladium, Richardia '. In the xylem only a few tracheides — 



' Compare Frank, Beitr. l.c. p. 138. 



" Compare Caspary, Berlin. Monatsberichte, 1862, l.c. — Trecul, Ann. Sci. Nat. 4 ser. tom. i, 

 p. 151- 



' Duchartre, Recherches sur la Colocase, Ann. Sci. Nat. 4 ser. XII. — Unger, Beitr. i.. Physiol. 



