RADIAL BUNDLES 



351 



rounded by three to five layers of rudimentary leptome-tissue, some of 

 the elements of which resemble sieve-tubes, while others correspond to 

 leptome-parenchyma (cambiform cells). When translocation is active, 

 particularly during the development of the sporogonium, this leptome- 

 tissue contains large quantities of protein-compounds. If translocation 

 is retarded or altogether inhibited, starch also appears in varying 

 quantities in the leptome tract. 



In the genus Pohjtrichum, and also in Pogonatwm aloidcs, the hadrome 

 of the erect aerial stem is entirely composed of water-conducting 

 tracheides ; but in Atrichum vn- 

 dulatum these are accompanied by 

 starch-containing conducting-paren- 

 chyma cells, just as in the case 

 of more highly organised plants. 

 In the subterranean rhizomatous 

 axis of Polytrichum, and in the 

 aerial stem of Dawsonia superba (a 

 Polytrichaceous Moss from New 

 Zealand), thick-walled mechanical 

 cells, which are directly comparable 

 to wood-fibres (libriform cells), occur 

 in association with the water-con- 

 ducting elements ; in these cases, 

 therefore, the rudimentary hadrome- 

 strand is equivalent to a xyleni- 

 bundle. The concentric axial vas- 

 cular bundle of the Polytrichaceae 

 is not sharply delimited from the 

 cortex. The entire absence of 

 bundle-sheaths comparable to those 



of the Ferns is, in fact, one of the features that indicate the rudimentary 

 character of the vascular bundles in Mosses. 



The radial bundles 1738, of roots are characterised as their name 

 implies by the radiate arrangement of the hadrome- and leptome- 

 strands. The hadrome is disposed in the form of more or less numerous 

 radial plates, which alternate with an equal number of isolated strips of 

 leptome. The intervals between successive " rays " of vascular tissue 

 are occupied by groups of conducting parenchyma, which are usually 

 about two cells in width. Generally speaking, the rays are more 

 numerous in stout roots than in slender ones. Among Dicotyledons 

 there are typically two, three, four, six or eight rays, while in Mono- 

 cotyledons their number may amount to twenty, fifty or even more. 

 With reference to the mode of development of the hadrome-plates, which 



Fig. 140. 



Part of a T.S. through the conducting strand of 

 the stem of Polytrichum juniperinum; r, cortical 

 parenchyma; I, leptome ; t lt thick-walled central ; 

 t, thin- walled peripheral portion of the hadrome. 



