COMPARATIVE ANATOMY OF AXGIOSPERMS 313 



phyUum, Gunnera, the Xymphaeaceae, etc. In the last-men- 

 tioned cases, the study of seedlings shows that the circular dis- 

 position of the fibrovascular strands is primitive. In Podo- 

 phyllum the scattering arrangement of the bundles is present 

 only in the aerial stem, and is absent in the rhizome, as well 

 as in the seedling. 



Fig. 113, EEj is from a photograph of one of the bundles 

 of Ranunculus acris. The bundle is surrounded by a scleren- 

 chymatous sheath, which is thickest externally. The xylem 

 and phloem are separated from one another by a narrow zone 

 of cells arranged in radial rows, indicating that a slight but 

 unmistakable cambial activity is present. The bundle is con- 

 sequently an open one. The protoxylem is obviously the inner- 

 most part of the primary xylem, so the bundle is endarch. En- 

 darch nbrovascular strands with secondary growth by means of 

 a cambium are characteristic of the Dicotyledons. In aquatic 

 Dicotyledons (e. g., the Xymphaeaceae), however, secondary 

 growth is frequently absent. 



The tracheary tissue of Dicotyledons with considerable sec- 

 ondary growth shows a further division of labor over the highest 

 Gymnosperms. In the oak, for example, there are thinner and 

 thicker-walled tracheids as well as vessels. The latter have 

 practically lost their water-conducting function and have very 

 few extremely small pits in their walls. They have thus been 

 differentiated for the purpose of support. In the beech this 

 division of labor among the tracheids does not take place, for 

 all the tracheids are of the same type and have well-developed 

 bordered pits in their walls. Strasburger 3 is of the opinion 

 that the wood-fibers of the Cupuliferae throughout are modified 

 tracheids, and hence merit the name of fiber-tracheids. Such 

 fibers are present in a number of the dicotyledonous orders. 

 In other cases, according to Strasburger, the wood-fibers are to 

 be regarded as derived from wood-parenchyma and not from 

 tracheids. In these instances they may properly be called libri- 

 form fibers. It is not clear, however, that a sharp distinction 

 can always be drawn between the two sorts of wood-fibers. 



The sieve-tissue of the Dicotyledons is also more highly 

 specialized than that of the Gymnosperms, for the sieve-tubes 

 have special accessory cells. These accessory cells are derived 

 from the same mother-cells as the sieve-tubes, and are known 



