STRUCTURE OF RADIAL BUNDLES. 



353 



I. In almost all Bicoiyledons where the point has been investigated, the original 

 bundle of the root is oligarch, usually with 2, 3, or 4 rays, more rarely with 6 or 8, 

 while higher numbers occur exception ally. In the main-roots the xylem-plate is usually 

 diarch-diametral, triarch, or tetrarch ; higher numbers occur rarely, whether in single 

 individuals (as 5-7 in specimens of Vicia Faba, and perhaps even 12^, instead of 4), 

 or as the rule for 'certain species, as most Amentacese (Quercus sp. 6-8, Alnus 5-6, 

 Castanea 6-12, Fagus 8, Carpinus 4), ^sculus (6), Coflfea (8), &c. None of these 

 numbers are constant absolutely and without exception even for the particular 

 species. Whether a definite number can be characteristic of one of the larger 

 genera or of a natural family (apart from occasional individual variations) is not 

 to be decided from the existing data. At any rate this is the case in several families 

 of which a dozen or half-a-dozen representatives have been investigated. Diarch 

 xylem-plates occur, for example," -in the main root of all investigated Cruciferae 



Fig. 163. — Ranunculus fluitans. Cross-section through the vasculai* bundle of a strong old adventitious rout <22S). 

 « endoderinis. p pericambium, s external primordial vessels of the diarch uniseriate xylem g—g \ between g—g and 

 p is the phloem. 



(Brassica, Raphanus), Fumaria, Caryophyllacese, Vitis, Urtica, Umbelliferas (Anthris- 

 cus Cerefolium, Foeniculum, Petroselinum sativum, Carum Carvi, Coriandrum, 

 Daucus, Pastinaca sativa v. Tieghem), Chenopodiacese (Beta, Atriplex, Spinacia), 

 Mirabilis, Centranthus, and Valeriana, and in Tagetes erecta among Compositae ; 

 tetrarch xylems as a rule occur in the investigated Cucurbitaceae (Cucumis, Cucur- 

 bita, Lagenaria, Luffa), Euphorbiaceas (Euphorbia, Ricinus, Mercurialis sp.), Tropse- 

 olum majus. Convolvulus tricolor ; generally the numbers 2 and 4 appear to be 

 predominant. But on the other hand considerable differences between the forms 

 investigated occur in the case of the higher numbers of the Cupuliferse above 



' Compare van Tieghem, I.e. p. 223. In the case cited it was doubtful whether the main root 

 or a strongly developed lateral root was in question. 



A a 



