^2Z PRIAfARy ARRANGE^fFXT OF TISSUES. 



one anotiicr in succession from wiiliin ouiwaids, willi gradual transitions, or with the 

 omission of one or the other intermediate form. As regards the occurrence of 

 particular forms of thickening, it may at any rate be given as a rule that among the 

 Monocotyledons the series ends with the development of dense fibrous thickenings, 

 annular and spiral fibres, reticular fibres, and surfaces with non-bordered pits or scalari- 

 form markings (pp. 1 58, 163). Bordered pits or scalariform slits here only occur in the 

 collateral bundles of stems which are long-lived and relatively slow-growing, as those 

 of many Palms, and Arundo Donax, and many rhizomes. The same often applies 

 to the stem and leaves of such Dicotyledons as form no secondary wood, though here 

 exceptions occur even in relatively short-lived parts, as for example in the leafy stems 

 of Thalictrum flavum and aquilegifolium. Tracheae with bordered pits appear in 

 the stem of most Dicotyledons and Gymnosperms which form secondary wood, at the 

 point where the latter joins on to the primary bundles (cf. Chap. XIV), and occur 

 further in the outer part of the bundle-trunks which traverse the leaf-stalks and leaves. 

 The elements of this part are in many cases ', especially among Coniferag, very 

 similar to those of the secondary wood in the stem generally, both in form and 

 structure ; yet this minute agreement is by no means of universal occurrence. 



As the structure of the walls changes from within outwards, the width of the 

 tracheae increases in comparison with that of the primitive elements, and this change 

 may take place according to the individual cases either gradually or suddenly, either 

 constantly or so that narrower tracheae again succeed the wider. 



In very small bundles, containing only one or two tracheae, the latter form a 

 group which, though varying in individual cases, does not require a further description. 

 On the other hand, larger bundle-trunks, which consist of numerous elements, 

 show remarkable differences both in their arrangement and in their gradually 

 changing width. 



The collateral bundle-trunks of most Dicotyledons and Gymnosperms show the 

 tracheae ranged in rows running from within outwards, which touch each otl;er 

 laterally, or are separated by rows of non-equivalent elements. In every row the 

 tracheae become wider towards the outside, and in thick bundles soon attain a- 

 average size, which remains uniform in the whole region (Figs. 157, 158, 183). I 

 the large bundles of many Dicotyledonous leaves the width of the elements first 

 increases successively, and then diminishes again to an average size, which further 

 towards the outside remains constant, e. g. leaves of Camellia, Ilex, Rosmarinus, 

 species of Eucalyptus, &c. The difference between the narrowest and widest 

 tracheae is in all these cases moderate, especially in comparison with many Mono- 

 cotyledons ; the largest is 2-3 times as large as the smallest, or even less. The 

 absolute size of the tracheae is also moderate ; in the leaves they are generally very 

 narrow, falling far short of the average width of the tracheides and fibres of the 

 secondary wood. Cf. the dimensions given in Sect. 40 and Chap. XIV. 



In the thicker bundles of the Monocotyledons other conditions are the rule. In 

 most cases the tracheae here form two main rows as seen in cross-section, which 

 diverge like the limbs of a V, At the point where the two limbs cut one another, or 

 inside it, lie the primitive elements. The end of each limb is usually occupied by a 



* Con., ... ; ;ank, Botan. Zeitg. 1864, pp. 167, 393. 



