392 PRIMARV APRANGEMENT OF TISSUES. 



case of the lateral leaf-trace bundles in the stem of Potamogeton crispus. It begins 

 here, as already indicated at p. 275 and in Fig. 125, with the appearance of the first 

 tracheides in the node, and advances from the latter, on the one hand, towards the 

 leaf, on the other hand basipetally in the adjoining internodes. Soon after the latter 

 basipetal development has begun, there appear, above the node, in connection with the 

 tracheides in it, the first tracheides of the portion of the lateral strand running 

 through the next internode, and these continue their development in the acropetal 

 direction. The two processes go on in opposite directions in every internode, 

 and meet about half-way up the latter, thus bringing the strand of primitive tracheides 

 into continuity ; the acropetal part starts earlier and grows slower than the basipetal. 

 The first origination of the bundles takes place, so far as could be decided, in 

 the acropetal direction. 



The completion of the leaf-trace bundles in Equisetum ' goes on, in a certain 

 sense, conversely to that in the case just described. It begins in the internode while 

 it is still very short, and in fact the primitive elements of the phloem first appear 

 rapidly in basipetal succession, then the first tracheides appear almost simultaneously 

 throughout the entire length of the internode. At a later period the development 

 of the primitive elements proceeds further and in fact upwards into the leaf, an^ 

 downwards to form the limbs, which are attached to the strands of the next loAver 

 internode. 



For the more complicated phenomena in the Phanerogams and Lycopods 

 possessing an axial bundle, and leaf-bundles laterally joining on to it, comp. 

 Sects. 70, 77, and 78, 107, 109, no; Nageli, /. r. pp. 38, 53, 56. 



Sect. 116. If a leaf-trace consists of more than one or two bundles, and if, as 

 in the great majority of cases, a median bundle is to be distinguished from lateral 

 ones, it is the rule for the median bundle to be first originated and completed, the 

 lateral bundles being the later developed the more distant they are from the median 

 one. Examples of this have already been given in Sect. 61. The converse order of 

 succession occurs but seldom ; as in the leaf-traces of Humulus ^, and of Phaseolus, 

 which have three bundles. Leaf-traces of Monocotyledonous plants with very 

 numerous bundles ranged in several rows in the node, behave, according to Nageli's 

 investigation of Chamaedorea and species of Cordyline, in agreement with the main 

 node ; but as regards the succession of the rows they diflfer in particular cases. Not 

 unfrequently the formation of the lateral bundles of a leaf-trace only starts when the 

 median bundles of several upper leaves are already present. If bundles of the leaf-trace 

 and cauline bundles are both present, the origination and completion of the former 

 takes place as a rule earlier in the same cross section, than that of the cauline 

 bundles. A conspicuous exception to this rule is presented by the Rhipsalideae 

 with winged stems ^. 



In recent times the question has been discussed, with reference to the original 

 differentiation of the meristem spoken of in the Introduction, what is the morpho- 



' Hofmeister, Vgl. Unters. p. 93. — Cramer, Pflanzenphys. Unters. Heft. 3, p. 26. — Nageli, 

 Beitr. I. p. 38. — Russow, Vgl. Unters. p. 145. 

 * Nageli, I.e. p. 114. 

 ' Vochting, /. c. 



