ANOMALOUS THICKENING IN DICOTYLEDONS AND GYMNOSPERMS. 593 



intermediate tissue. The arrangement of the latter is, at least in extreme cases, 

 different according to the presence of successively renewed rings of cambium and 

 thickening, or of a permanent extrafascicular cambium. 



In the first case the bundles are arranged like the strands of wood or bast of a 

 normal Dicotyledonous ring. One strand of bast corresponds to each strand of 

 wood, the two together forming one collateral vascular bundle. Between two 

 bundles of a ring lies a radial band of intermediate tissue, which, with reference 

 to the ring itself, is like a normal medullary ray ; each pair of successive rings is 

 divided by a zone of intermediate tissue. As will be shown below by examples, 

 these zones derive their origin from the outer margin of the zone of bast bordering 

 them internally, or the parenchyma directly adjoining this margin externally. The 

 bundles of successive rings are connected one with another by branches with an 

 obliquely radial course, the arrangement of which varies according to individual 

 cases to be described below. The number of the successively alternating zones 

 of bundles and intermediate tissue has no constant relation to that of the annual 

 periods of growth, but rather in one period of vegetation a number of successive 

 zones without definite limit are formed. 



In the seco nd case all bundles lie in the wood, and alternate both in radial and 

 tangentiaTdirection with intermediate tissue, either so that they appear in a transverse 

 seQtioh distributed quite irregularly_in^ the latter, as e. g. in the thick masses of wood 

 of shrubby and tree-like Chenopodiacese (Halimos, Arthrocnemum), Nyctagineje 

 (Bougainvillea, Pisonia ^), Amarantacese (Aerva, Pupalia "), and Mesembryanthema ; 

 or that they^appear_injjransveise section arrangedjnjrregularly concentric zones 

 within the intermedia te tissue, as in Salsolai. Haloxylon, and Caroxylon ; in the root, 

 ancJ^artly also^in the stems of Mirabilis. It is doubtful how far these zones are related 

 to the aiiHual production in woody stems; inJMirabilis there is no surfi relation. 

 In their longitudinal course the bundles show mutual connections both in a radial 

 and1angential_direction ; in the stem of Mirabilis these are at the nodes; in the 

 otherinvestigated cases also at other points, since, in their course, which undulates in 

 both directions, they alternately approach and recede from one another, thus forming 

 elongated and pointed meshes. 



" The histological composition of the vascular bundles is in general like that of 

 collateral bundles, with the limitation that narrow spiral and annular vessels are 

 usually restricted to the bundles of the leaf-trace; only the inmost, apparently 

 medullary secondary bundles of Mirabilis and other Nyctagineae have, like these, 

 spiral vessels also. In the other secondary bundles the xylem consists of pitted 

 vessels, and in sappy and flesh3c,parts also of reticulate vessels, and of parenchymatous 

 and fibrous elements which require further investigation. Between the two latter the 

 vessels are arranged usually in one or a few, more or less regular radial rows ; more 

 rarely one or another vessel is at a distance from the rest in the intermediate tissue. 

 In many Mesembryanthema the arrangement seems to be less regular, so that the 

 vessels appear. 'irregularly scattered' in the intermediate tissue'. The phloem in 



' [See Petersen, v.. Hist. u. Entw. des Stengels der Nyctagineen, Ref. Botan. Zeitg. 1880, p. 509.] 

 '' Under the generic names' quoted it is intended in each case to include specially the above- 

 namedjpecies belonging to them. '^ Compare Falkenberg, /. c. 



