STRUCTURE OF COLLATERAL BUNDLES. 323 



trachea, exceeding the primitive elements many times in width, with dense spiral, or 

 narrow reticular thickenings on its wall ; and this either forms the end of a continuous 

 or interrupted row of successively wider tracheae, or it follows suddenly on much 

 narrower ones. In the middle between the two limbs there are either no vessels 

 (even the whole Phloem may be included here, e, g. the leafy stem of Asparagus \ 

 and Tamus communis), or the middle is occupied and more or less filled up by a 

 group of narrow, densely reticulated or pitted vessels, as for example in the Grasses 

 (Fig. 151); and this group may spread out even beyond the external edge of the 

 large tracheae at the ends of the limbs (Fig. 147). In the laterally flattened Mono- 

 cotyledonous bundles mentioned above, the tracheae lie in an interrupted single, or 

 in places multiple row, run'^ing from within outwards. In this it is usual for one or 

 a few narrow primitive elements to be followed on the outside by one or a few 

 tracheae of considerable width, e. g. by a very large spiral tracheide in the leaf- 

 stalk of Musa ^ and Canna, &c. ; further outside there are either no more tracheae, 

 e. g. in the leaf of Pandanus ^, or some relatively very narrow ones, e. g. Musa, 

 Canna, Helicon ia, &c. : the broad bundles in the stem of many Palms, especially 

 Calamus', though not laterally flattened, also show the same character ; in Calamus 

 some narrow spiral vessels are followed by a single pitted vessel of enormous width 

 (cf. p. 169), and there are no others further outside. 



The phenomena mentioned give rise to the characteristic habit of most Mono- 

 cotyledonous bundles, which is especially evident in cross-section. They occur 

 also among those members of the class which, like the Dioscoreae, behave differently 

 to the others with respect to the arrangement of the bundles (cf. p. 276). On the one 

 hand, however, they are not confined to Monocotyledons, for the bundles in species 

 of Ranunculus, and especially of Thalictrum, belong to or are closely connected with 

 the form first described for Monocotyledons, while those of Neh\mbium stand in the 

 same relation to the second form. On the other hand, their distribution is by no 

 means universal even in those families of Monocotyledons in which they prevail. 

 The cross-section through the bundle-trunks in the leafy stem of Fritillaria im- 

 '"^ perialis and in the leaf of Phormium tenax '" shows, for example, a triangular group 

 ■"'(widening towards the outside) of moderately wide tracheae, which increase but 

 litde in size towards the outside, and show the regular character and succession in the 

 structure of their walls. The thicker bundle-trunks in the leaf of Yucca filamentosa 

 (from which the thinner ones only differ in the number and size of their elements) 

 show a thick xylem, broadly triangular in cross-section, in which the primitive ele- 

 ments are succeeded by a likewise broadly triangular group of spiral vessels with a 

 closely-wound fibre, which are of moderate and tolerably uniform width. With this 

 group is immediately connected on the outside a zone, consisting of about four cross- 

 rows of narrower reticulated and pitted vessels, united at all points, which on the other 

 side borders on the phloem. 



The bundles of the Equiseta, to be further described below, agree essentially 



' Von Mohl, I.e. Tab. G. ^ Von Mohl, I.e. Tab. G, fig. 3. 



» Von Meyen, Phytotomie, Taf. VIII. * Von Mohl, /. c. Tab. D, F. 



5 Von Mohl, /.r. Tab. G. 



Y 2 



