364 PRIMARY ARRANGEMENT OF TISSUES. 



root-Structure ; their histological peculiarities are essentially similar to those of the 

 stem of the same plant, and like the latter still require more exact investigation. 



The pericambium appears, as a rule, as a single layer all round, but it also 

 occurs with two layers ; this is the case only outside the phloem-groups in Aspidium 

 Thelypteris, and all round in Polypodium ireoides^; in Osmunda and Todea there are 

 several layers all round. In the Equiseta, in contrast to the other forms belonging 

 to this series, all the cells of the pericambium stand precisely in front of those of the 

 endodermal sheath, and together with these have arisen from the division of the 

 innermost cortical layer. In the other vascular Cryptogams the latter forms the 

 endodermis only, while the pericambium originates by tangential division of the 

 plerome-cylinder surrounded by the cortex. 



In the endodermis, apart from various subordinate differences of form, the cells 

 lying in front of the corners of the xylem-plates are the initial cells of the lateral 

 roots, and are often distinguished from the others by their more considerable size. 

 The structure of the endodermis is otherwise essentially the same as in the bundles 

 of the stems of the same plants. 



In the roots of Cryptogams, the orientation of the diametrally diarch xylem- 

 plates is always such that their surface cuts the median plane of the next higher 

 order of ramification at right angles. Those arising on the stem appear, according 

 to species, either to have the like orientation with reference to its median plane, or 

 to have their surface coincident with the median- plane of the stem. 



The axial root-bundles of the Marattiacese ^ are distinguished from those of the 

 other Ferns, with which their structure otherwise agrees, by their tetrarch or polyarch 

 xylem ^. The number and length of the radial plates increases, in the same species, 

 with the thickness of the roots; the former may amount to 18-20. In the thicker 

 roots we frequently find them converging in pairs, and as seen in cross-section united 

 to form the figure V. In the roots occurring above the ground the xylem-plates do 

 not reach to the middle of the bundle ; in the thin, 4-5 rayed branches underground, 

 they meet, according to Russow, in the middle. 



The very thin root-bundle of Azolla *, which differs in its development from 

 that of the Ferns, has, according to Strasburger, a usually triarch xylem, consisting 

 only of spiral tracheides. Besides this there are only some inconspicuous elements 

 lying inside the pericambium, and constituting a doubtful phloem. 



Sect. 109. A structure departing from the general radial type of root occurs in 

 the rhizophores of Selaginellce ■^ in the true roots of the same plants, in the thinner 

 roots of Lycopodia, and in the roots of Isoetes and Ophioglossum ; with the excep- 

 tion of the rhizophore of Selaginella Kraussiana, the peculiarity of this structure 

 consists in the fact that the usually monarch xylem either occupies one side of the 

 bundle, the phloem lying on the other— the arrangement being thus collateral — or 

 that the former is at least strongly approximated to one edge of the phloem which 

 surrounds it. jNIost roots or rhizophores belonging to this series are dichotomously 



' Van Tieghem, /. c. — Compare also Nageli und Leitgeb, p. 83. 



■■* Meyen, Haarlemer Preisschiift (1836), Tab. VIII. 



' [Cf. Holle, Konigl. Ges. d. Wiss. zu Gott. Jan. 8, 1876.] 



* Strasburger, Ueber Azolla, p. 48. 



' [M. Treub, Recherches sur les organes de la Veg. du Selaginella Martensii. Lejden. 1877.] 



