GYMNOSPERMAE. 



343 



bundles in the leaves of the Cycadeae is obviously very like that of many Ferns (see 

 on page 313). 



The woody body of the stem is formed of the descending Icaf-tracc-bundles, which 

 are at first completely isolated, but are soon united into a closed ring by the cambium 

 which bridges over the medullary rays. The protoxylem, the medullary sheath 

 in older stems, which consists of the xylem of the several leaf-trace-bundles, contains 

 in all Gymnosperms, as in Dicotyledons, long narrow elements with annular or spiral 

 thickening-bands, and towards the outside reticulately thickened or scalariform elements. 

 The secondary wood produced by the cambium-ring after growth in length has ceased 



\l3 \lo\ IS \/7\/7 9 I/* U/ 



Fig. 264. Pinus sylvestris. Diagrammatic repre- 

 sentation of the course of the bundles in the young 

 shoot, the surface of the cylinder being expanded into 

 a plane. Leaves in a right-handed spiral with a diver- 

 gence of g. The numbers indicate the leaf-trace- 

 bundles in their order of succession which appear as 

 broad bands. The bundles which converge in pairs, 

 shown by the thin lines by the side of the prominent 

 leaf-trace-bundles o — 9, run each pair to an axillary 

 shoot. The foliar bundles unite each with the eighth 

 below it. From De Bary, Vergl. Anatomic, after 

 Geyler. 



Fig. 263. Pinus sylvestris. Radial longitudinal section through 

 the wood of a vigorous branch ; c cambial wood-cells, / /' t" bordered 

 pits of older wood-cells (tracheides), s large pits where medullary 

 rays are in contact with the wood-cells. 



consists in the Coniferae of long prosenchymatic tracheides having a few large 

 bordered pits, which in later-formed wood at least are usually circular ; between these 

 tracheides and the spiral vessels of the medullary sheath all possible intermediate 

 forms may be found. The characteristic difference between the secondary wood of 

 the Coniferae and that of the Dicotyledons is that the former has only this prosen- 

 chymatous' form of cell, and none therefore of the wide dotted shortly articulated 

 vessels which traverse the compact narrow-celled woody mass of the Dicotyledons. 

 On the other hand, this predominance of prosenchymatous tracheides recalls the 



Wood-parenchyma is either not formed at all or only in small quantities. 



