STRUCTURE OF RADIAL BUNDLES. 



349 



seen delicate sieve-pores. In the subterranean shoots of the rhizome (NageU and 

 Leitgeb's rhizoides) the bundle is very weak and rudimentary in its development ; I 

 find only a flat or three-cornered xylem, consisting of a few, frequently only 3-6, re- 

 ticulated and scalariform tracheides, separated here and thereby thin-walled elements; 

 the peripheral tracheides are but little narrower than the internal ones ; the xylem is 

 completely surrounded by 2-4 layers of delicate spindle-shaped cells. I could see 

 nothing of any sieve-tubes. The vascular bundle of Tmesipteris appears from 

 Russow's statement to have a similar structure to that of Psilotum. 



In the cylindrical axial strand of the stems of Lycopodium (comp. p. 281) the 

 xylem consists of a number of plates or bands, the peripheral corners of which are 

 each formed by a group of narrow tracheides (comp. p. 163), the above-described 

 points of attachment of the leaf-trace bundles, while the larger inner part consists 

 of wider scalariform tracheides. (Comp. Fig. 162.) The number and arrangement 

 of the plates and their relations to 

 the rows of leaves vary with the 

 species, and with the vigour of the 

 individual shoots. Of the latter 

 relations we have already spoken 

 above. As regards the other con- 

 ditions which come under con- 

 sideration \ among the native species 

 investigated L. inundatum has 3-5 

 plates, united in the middle to form 

 a body which has a stellate cross- 

 section, thus constituting a tri- to 

 pentarch radial xylem. The latter 

 is however even here not unfre- 

 quently in so far irregular, that one 

 or the other plate separates from 

 the rest, so as to have an isolated 

 course for some distance, and then again to unite with the others. Four radial plates 

 united in the middle are present in the ultimate ramifications of the heterophyllous 

 species, as L. complanatum, and as a rule L. alpinum, though here deviations occur in 

 20-30 per cent, of the cases. In the stouter axes of the last-named species, as well as 

 in L. clavatum, annotinum, and Selago, the number of the vascular plates is higher, 

 being in proportion to the thickness of the shoots ; — in stout main stems of L. com- 

 planatum and alpinum, for example, it amounts to 11 and 13, in those of L. anno- 

 tinum and clavatum to 17, but diminishes again in the weaker ramifications to 4 and 3. 

 In these cases the plates are only partially, or scarcely at all radially convergent ; most 

 of them rather form separate bands in the decidedly bilateral prostrate main-shoots 

 of all species possessing them (Fig. 162), these bands being slightly curved, with 

 their convex surface always directed towards the lower side of the stem, and their 

 corners lying chiefly right and left ; they are further united with each other in a great 

 variety of ways, sometimes radially, sometimes so as to form loops. Their union and 



F[G. 162. — Lycopodium Chaiii.-Ecyparissus. Cross section of a shoot, 

 ma-fnified about lOo times. In the middle the cylindrical vascular strand ; 

 in the dense cortex to the right the cross-section of a bundle running into a 

 leaf. From Sachs' Textbook. 



^ Hegelmaier, /. c. p. 790. 



