356 



PRIMARY ARRANGEMENT OF TISSUES. 



2.. The axial bundle of the root in the Gymnosperms^ is in general similarly con- 

 structed to the ordinary one of Dicotyledons. Its original structure is always altered 

 very early by secondary growth from the cambium ; the sclerenchymatous fibres in the 

 periphery of the phloem region of Dion described by Reinke may have owed their 

 origin to this. Over the angles of the xylem-plates the pericambium is single-layered 

 in Taxus, Thuja, and Biota ; many-layered (from 3 and 4 to 7 cell-layers thick) in 

 species of Podocarpus, Pinus, and in the Cycadese investigated. 



The xylem-plates consist at their outer corners of tracheides, with the fibrous 

 thickenings generally characteristic of this region ; in their internal, later developed 

 portion they consist of pitted tracheides, such as are characteristic of the wood 

 of Gymnosperms. 



Among the Goniferse the Cupressinek and Taxinese have diametral and diarch xylem- 

 plates in roots of all degrees, or more rarely triarch ones. In the Abietinese higher 



Fig. z65 (145) ■ — Ranunculus repens. CrosS'Section through the vascular bundle .of an old adventitious root. 

 M endodermis, fi pericambial layer, g external primordial vessels of the tetrarch xylem, r larffe axial pitted 

 vessel. In the pitted vessel X surface-view of a roundly perforated cross-wall. A narrow zone of secondary 

 wood has been deposited on the primary xylem plates extending from ff\or; the cells between this and the 

 phloem groups are tangentially divided ; cf, Chapter XIV. 



numbers and with them greater individual variations are the rule, though here no con- 

 stant relation exists in the main root between these variations and the number of the 

 cotyledons, which, as is well known, is likewise variable and always more than two. In 

 Abies excelsa, for example, van Tieghem found in 1 3 seedlings a triarch root-bundle, the 

 cotyledons numbering 7, 8, 9, or 10 ; in a specimen with 6 cotyledons the bundle was 

 diarch, in one with 8, tetrarch. The numerous investigations of the observer mentioned 

 established similar relations for the species of the genus Pinus in the narrowest sense 

 (P. Pinea, halepensis, sylvestris, &c.). The number.of the xylem-plates here amounts 



' See van Tieghem, /. c. — Strasburger, Coniferen und Gnetaceen, pp. 340, 360, Sec. — Mettenins, 

 Beitr. z, Anatomie d. Cycadeen, p. 595, &c. — Reinke, Morpholog. Abhandl. I. 



