6lO SECONDARY CHANGES. 



selves; along the periphery of the stem they connect the two curves belonging to 

 one leaf on the side opposite the latter into a ring-like transverse girdle, open only 

 at tTie two ends which enter the leaf: this girdle may frequently be closed even 

 between the points of exit by a transverse anastomosis. Simultaneously with the 

 connections described, anastomoses further appear in the second place between 

 neighbouring girdles, their number and arrangement not being closely defined ; others 

 also appear, which run generally in a radial direction from the girdles, inwards and 

 obliquely downwards, and insert themselves on the bundles of the primary bundle-ring. 

 In Zamia muricata, according to Mettenius, these radial connections run unbranched 

 and moderately straight from the ring of bundles to the girdles. In Cycas revoluta, 

 Dion, and Encephalartos horridus they divide, as they pass from the bundle-ring, 

 usually into two diverging branches, which branch further, and sometimes anastomose 

 mutually with their branches, and sometimes pass on to the girdles. The whole of 

 these branches form a complex and irregular cortical net-work of bundles (Fig. 240); 

 In a specimen of Cycas revoluta Mettenius found that nine to eleven of the bundles 

 or bundle-trunks that leave the ring belong to one leaf. 



The average strength of the girdles, and of the radial connections and their branches, 

 the special direction of their course, &c., show many variations, which cannot be 

 enumerated here, partly according to species and individuals, partly, and especially 

 in Dion, according as they belong to foliage or scale leaves. In strong stems of 

 Cycas or Encephalartos the absolute thickness of the strongest reaches I™™ and 

 more. All the bundles of this original system, which may be called the primary 

 net-work of bundles, are collateral ; tracheae and sieve-tubes are arranged between 

 radial rows of parenchyma in radial rows, which are themselves interrupted by 

 parenchyma ; the tracheal elements at the medullary margin are often subsequently 

 torn apart by the expansion of the parenchyma. All tracheal elements, as has 

 already been noted on pp. 165 and 336, are tracheides ; the innermost primordial 

 tracheides are spirally thickened, the majority have transverse scalariform pits. 



As far as is known the primary net-work of bundles is developed in its 

 full complexity close below the end of the stem. The successive transverse portions 

 of the stem undergo, it is true, a considerable growth in thickness after the moment 

 when it is already completed, in the first place by the still continued expansion 

 of the parenchyma of pith and cortex, and subsequently by the cambiogenetic 

 increase to be described later. The primary net remains meanwhile permanent; 

 both the radial connections, 'and especially the girdles, must therefore increase 

 continuously in length. As Mettenius found, the spiral tracheides do not take 

 part in this, they are t^rn apart and finally cannot be recognised. The scalariform 

 tracheides grow, on the other hand, considerably in length; in the girdles of young 

 leaves the shortest measured 0.09mm; jn those of older stems the length increased 

 to I •4mm, in the oldest stems investigated to 4-5'""". The unthickened points of 

 the walls are thus expanded from the form of narrow transverse slits to broad 

 elliptical pits. 



In the primary ring of bundles a cambial zone arises, as far as is known, 

 according to the normal mode for Dicotyledons and Gymnosperms, and this also 

 produces wood and bast generally in the normal way. The strands of xylem and 

 phloem are situated in the transverse section between broad multiseriate large me- 



