I9 o8] THIESSEN—DIOON EDULE 375 



tal wood is primary and centrifugal wood secondary does not hold 

 true, a mistake apparently shared by Matte. It has been shown 

 that protoxylem and metaxylem may have both centripetal and cen- 

 trifugal elements. This is very well seen in cross-sections at a low 

 level of a young strand, where only a few xylem elements are devel- 

 oped. In such a section the protoxylem lies against the inner edge 

 of the bundle, and the procambium can be seen to occupy considerably 

 more than half of the bundle; the boundary of the procambium and 

 the protophloem is distinctly recognizable, the protophloem forming 

 only about one-third of the bundle. In the upper extremities of the 

 petiole, also, there may be seen at an early stage a considerable amount 

 of procambium outside the protoxylem; as these usually fail to de- 

 velop xylem, thin-walled cells occur between the primary and second- 

 ary wood in the upper extremities (fig. 21). 



The girdling habit was first noticed and described by Karsten 

 (2) in Zamia muricata^ithoui, however, giving the definite number 

 of traces. Lestiboudois (3) adds nothing new except that the 

 traces branch and anastomose. Mettenius (4) next misinter- 

 preted the situation, as described in the historical introduction 



(P- 357). 



In the embryo and young seedling, at least, the leaf traces pursue 



definite and well-defined courses and constitute a definite system. 

 Four traces are invariably found to leave the vascular cylinder for 

 each leaf. A few anastomoses occur here and there, but these are 

 always reducible to four strands. On two occasions only five strands 

 were found in one of the cotyledons. In the older seedlings anas- 

 tomoses are more abundant, but so far as observed these can be 

 reduced to the system found in the young seedling. 



When Karsten (2) described the girdling habit, he suggested a 

 cause for it in saying that the bundles are formed very early in the 

 young leaf, and that the originally narrow curves are later crowded 

 far out by subsequent growth and the appearance of new organs. 

 Mettenius (4) also gives a reason for girdling as follows: "In the 

 developmental stage the traces of the youngest leaves lie in the region 

 of the vegetative point, and at first ascend in an almost perpendicular 

 direction, but during the further growth assume gradually an almost 

 horizontal position, and with subsequent growth are lengthened and 



