i6oo A TEXTBOOK OF THEORETICAL BOTANY 



through 1 80° C. so that their exterior point (the protoxylem) is now 

 directed inwards. Each half attaches itself to one of the phloem bundles 

 to form a duplex, collateral bundle. There are thus in the hypocotyl as 

 many bundles as there were phloem bundles in the root. 



Type 2. The phloem bundles divide, like the xylem bundles, and the 

 halves separate and place themselves facing the halves of the xylem bundles. 

 There are thus twice as many bundles in the hypocotyl as there are phloems 

 in the root and the medullary rays are narrower than in Type i. d 



Type 3. Both xylem and phloem bundles divide, but the xylem bundles 

 remain in place, turning round, as before, and it is the phloem halves 

 which move across to unite with the xylem halves. 



Normally the transition level lies at the collet and the change is passed 

 through in a short vertical distance, but intercalary growth either above or 

 below may cause an apparent displacement either up or down from this 

 level, though the external collet does not move. 



Actual sections of seedlings often show inconsistencies with Van Tieg- 

 hem's simple, one might say geometrical, schemes. One outstanding dis- 

 crepancy is that as the xylem halves separate there is often seen a median 

 protoxylem group which is not attached to either of them. Van Tieghem 

 evidently thought of the hypocotyl above the transition level as equivalent 

 to an epicotyledonary internode, but things are not quite so simple. Before 

 the epicotyl is reached, the cotyledonary node is passed through and the 

 cotyledonary traces are of a special type, which has a marked effect on the 

 vascular structure from which the traces come. So concerned have been 

 the later workers with the cotyledonary traces that one might imagine the 

 epicotyl to be non-vascular for all they say about it. Miss Thomas laid 

 stress on the fact that the cotyledonary midrib is either V-shaped in section 

 or composed of two divergent halves, in either case with a common 

 protoxylem between them in the endarch position. It is really a triad 

 rather than a dyad structure, for this protoxylem is the independent 

 protoxylem which we saw between the xylem halves at the transition 

 level. As the node is approached from below, two of the phloem-xylem 

 bundles in the hypocotyl come together and unite to the intermediate 

 protoxylem to form the midrib of the cotyledon. There are also lateral 

 bundles in the cotyledon and these also may sometimes originate well down 

 the hypocotyledonary stele. 



Chauveaud performed a service by pointing out, in this connection, 

 that it is not actually the vascular bundles which "move" or "turn", 

 an impossible feat, but the positions in which differentiation is taking 

 place. Instead of vessels or sieve tubes continuing upwards in a vertical 

 line, the locus of differentiation moves by degrees in one direction or 

 another so that the succeeding elements are displaced relative to the 

 earlier ones, thus bringing about the appearance of bundle movement. 

 He considers the change from alternate vascular tissues to the superposed 

 arrangement to be a repetition of a phylogenetic change. The root shows 

 the primitive arrangement, which changes to the later disposition at higher 



