352 PRIMARy ARRANGEME.WT OF TISSUES. 



For the structure of the individual parts few general rules are to be given^, 

 except those which hold good generally for vascular bundles and their sheaths. The 

 xylem-plates consist of one or more rows, which, according to the particular case, 

 are uninterrupted in the radial direction, i. e. one trachea follows on another ; or 

 they are interrupted by the interposition of non-equivalent (parenchymatous or 

 sclerenchymatous) elements. For the special nature of the trachese, i. e. whether 

 they are vessels in the strict sense or tracheides, the rules and difficulties stated at 

 p. 164 apply. The first-formed vessels or tracheides, which occupy the corners, are 

 always narrow, the later ones, following in a centripetal direction, become suddenly or 

 successively wider. The latter are always pitted or reticulated vessels (or tracheides); 

 the narrow peripheral ones are as a rule also reticulated or annular vessels, with 

 dense and fine thickening fibres, the prevalent direction of which is transverse. For 

 short distances however the fibre has not uncommonly in these cases also a simply 

 spiral course. Closely wound spiral fibres, which can be unrolled for a long distance, 

 occur more rarely, e. g. in the roots of Tornelia fragrans, Cucurbitacese, Anthriscus 

 Cerefolium (Van Tieghem), Phaseolus (Dodel), Cycadese (Mettenius), and Coniferae. 



The structure of the phloem-rays, where they are well developed, is essentially 

 the same as in the typical collateral or concentric bundles. In feeble roots of Mono- 

 cotyledons they are not uncommonly reduced to one sieve-tube with narrow-celled 

 surrounding tissue (e. g. Triglochin maritimum, Aponogeton, Hydrocleis Humboldlii, 

 Potamogeton lucens, comp. Van Tieghem, /. c. Taf. VI), this being of t3^pical structure, 

 only small. It is therefore to be supposed that the typical structure belongs to them 

 generally, though they still require more exact investigation, especially in the small- 

 celled bundles of Dicotyledons. I should also wish to extend the last remark to the 

 roots of Coniferee, in the primary bundles of which, according to Janczewski's more 

 recent statement \ sieve tubes are said to be wholly wanting. 



The number, and with it also the arrangement and relative breadth of the xylem 

 and phloem-rays, the extent and distribution of the tissue occurring around and 

 between them, lastly the special structure of the particular forms of tissue, and thus 

 the entire structure of the root-bundle, vary, sometimes in different roots of the same 

 species, sometimes according to the species and the larger systematic divisions. In 

 the former relation the general rule holds good that as the thickness of the roots 

 diminishes, not only does the number of the tissue-elements in the bundle diminish, 

 but also the number of its radial plates, if in the thicker specimens this exceeds two. 

 Further slight individual differences, which cannot be referred to difference in thick- 

 ness, occur among members of the same species. In the other relation, besides the 

 obvious identity or similarity of structure of closely related forms with similar adap- 

 tation, the great conformity of structural plan in all divisions of vascular plants is to 

 be emphasised. For none of them can a special structure be stated as everywhere 

 characteristic of the group. Van Tieghem's first plate shows the almost identical 

 cross-sections of young roots of Cyathea medullaris, Allium Cepa (main root of the 

 seedling), Taxus, and Beta. Smaller diff'erences between subdivisions of the larger 

 classes are often more sharjjly expressed. The existing investigations give rise to the 

 following rules : — 



'o 



' Ann. Sci. Nat. 5 ser. torn. XX. p. 31. 



