STRUCTURE OF RADIAL BUNDLES. ^S7 



to 5-6, rarely 7. They are distinguished from those of the closely-related Abietineae by 

 their form, which may be compared to that of a Y. Each of them is, literally speaking, 

 diarch ; and begins externally with two rows of about five narrow tracheides, touching 

 the pericambium at two separate points ; they converge towards the inside and abut on 

 each other. From their point of junction a radial row of tracheides, 1-2 layers thick, 

 extends in the centripetal direction, without reaching the centre of the root. In the 

 angle of the Y lies a resin-canal surrounded by delicate cells. 



The roots of Ephedra which have been investigated have diametrally diarch xylem-plates. 

 Among the Cycadeae the xylem in the subsidiary and branch-roots, which havfe 

 been investigated in numerous species, is usually diametrally diarch, the two original 

 plates meeting in the middle, or being separated by parenchyma. The same holds good 

 for the investigated main roots of Cycas revoluta and Zamia furfuracea. More rarely, 

 in the thick subsidiary roots of usually diarch species, the bundles are three-rayed. In a 

 hybrid Ceratozamia van Tieghem found three or four xylem-plates, and in a specimen of 

 Zamia muricata Mettenius found six in the main root. In the subsidiary roots of Cycas 

 revoluta, when the centripetal development of the plate is already advanced, some 

 scattered narrow reticulated vessels appear at the sides of its peripheral corners, as if 

 secondarily ; whether these constitute the first beginnings of the secondary growth re- 

 mains to be decided. 



3. Among the Monocotyledons there are first of all many thin main roots of the 

 seedling, which in the structure of their axial vascular bundle are indistinguishable 

 from those of Dicotyledons and Gymnosperms. In the case of Allium Cepa, with a 

 diametrally diarch, and sometimes Iriarch xylem-plate, this has already been mentioned 

 above ; A. Porrum and Lilium Martagon are characterised by a similar structure of the 

 main-root, Tulipa Gesneriana shows the deviation that its pericambium consists of 

 two layers instead of one. Bulbine annua has three xylem-plates, which do not meet ; 

 Iris jNIonnieri has four. The weaker roots of all degrees are essentially similar to 

 those just described. 



Stouter main-roots, such even as those of species of Asphodelus, Canna, and 

 Asparagus officinalis, and then those of the Palms (Phcenix, Seaforthia elegans), and 

 above all the subsidiary roots springing from the stem (which in this class, as is well 

 known, usually far exceed the main-roots in thickness), though in the great majority 

 of cases they maintain the typical plan of structure, yet become polyarch as their 

 bundles increase in bulk, and also show a more varied differentiation, due to differ- 

 ences of many kinds in the structure of the tissue-elements. Comp. Figs. 166, 

 167, 168. 



First of all, as regards the number, arrangement, and form of the groups of tissue 

 of these typical roots of INIonocotyledons, the number of the xylem and phloem rays 

 rises from 5-10 up to 20, 50, and more. The thick roots of Iris, Asparagus, 

 Smilax (Sarsaparilla), Palms \ &c., are examples of a high degree of polyarchy. The 

 phloem-bands are always small, consisting of relatively few elements, their cross- 

 section being roundish or radially elongated. The xylem-bands, consisting of one 

 or a few rows of elements, usually begin at the periphery with a short uninterrupted 

 radial band of narrow tracheae, which become gradually wider towards the inside. 

 These are suddenly followed in the centripetal direction by one or a few, very wide, 

 reticulated or pitted vessels. The latter are usually separated from the peripheral 



' Von Mohl, Palm. Structura, Diplothemium maiitimum, Tab. I. 



