ANOMALOUS THICKENING IN DICOTFLEDONS AND GFMNOSPERMS. 6ll 



dullary rays, which form, when their longitudinal course is seen from without, 

 acute meshes between the undulating bundles. Through these meshes all the 

 leaf-traces and radial connections of the primary net-work of bundles. pass between 

 the secondary bundles, in their course from their internal points of insertion into the 

 cortex. The parenchyma of the medullary rays, which surrounds them, consists 

 of large radially elongated cells, which undergo only few tangential divisions as 

 the thickening proceeds, so that one may almost speak of an interruption of the 

 cambial layer by large-celled parenchyma aa the points in question. It may 

 be concluded even from these facts, and it is confirmed by further investigation, 

 that the cambial zone arises in the lower ends of the bundles of the leaf-trace, 

 which descend perpendicularly, and that the upper parts of them, which curve into 

 the ring, take no part in its formation. Further it remains to be more exacdy 

 investigated, how far the radial connections share in the formation of the ring and 

 cambium, and whether other intercalary bundles are formed in addition. 



After it has once appeared the zone of cambium forms wood and bast in the 

 normal way. Both retain a regular arrangement in radial rows, and both tracheae and 

 sieve-tubes are disposed in numerous narrow radial rows, consisting of one or more 

 series, and interrupted by thin-walled parenchyma : these lie between relatively 

 broad, but small, parenchymatous medullary rays. The tracheal elements of the 

 secondary wood are exclusively Iracheides of moderate width, and having on 

 their radial walls several rows of transversely elongated bordered pits (Cycas, 

 Encephalartos), or with a scalariform-reticulate wall (Zamia spec, Stangeria). The 

 sieve-tubes have been treated of on p. 179. They are accompanied in Cycas, 

 Dion, and Encephalartos by small groups of hard sclerenchymatous fibres, which are 

 absent in Zamia and Stangeria ; besides these Mettenius describes isolated chambered 

 sacs containing klinorrhombic crystals. 



The whole mass of secondary wood and bast has accordingly, in the main, the 

 structure typical of sappy, parenchymatous stems of Dicotyledons. It remains 

 always narrow in comparison with the pith and cortex ; the bast is usually strongly 

 developed relatively to the wood, and is often of equal thickness with it. 



The investigated species of the genera Zamia, Dion, and also of Stangeria, 

 show throughout their life the structure hitherto described, i. e. the primary network 

 of bundles, and the normal ring of secondary thickening, which grows in thickness 

 slowly and without limit. There are no other parts added to those described. But 

 in the species of Cycas and Encephalartos the case is otherwise. Firstly, since in all 

 of them the growth in thickness of the first normal ring is limited, it ceases after 

 a time not exactly defined, but certainly longer than one period of vegetation, and is 

 continued by a reneived cambial zone, which appears in the cortical parenchyma 

 at the outer limit of the bast-layer — fundamentally in the manner described above in 

 Sect. 191. The renewal of the ring may subsequently be repeated more than once. 

 Miquel's large stem of Cycas, which was certainly many years old, had e.g. 6-8 

 successive rings. 



Both the coarser structure of these rings and their histological composition are 

 the same as in the first. Also the transit of the radial connections of the primary 

 net through the medullary rays is conducted in the same way as in the former case. 

 As in most cases of successively renewed rings, which were described in earlier 



R r 2 



