592 
E antes . — On the Occurrence of 
The development of a bundle in E . hyemale var. robustum is shown in 
Fig. 2. In this case, as in that of the birch wood, necessary precautions 
were taken in the preparation of the section. The preparation photo- 
graphed came from near the centre of a good-sized piece of stem embedded 
in celloidin, which was not removed. Here is seen a condition which is 
apparently most significant. For in the rows of metaxylem-cells, which 
can be clearly made out flanking the carinal canal on either side, differentia- 
tion, as shown by loss of protoplasm, is seen to be centrifugal upon the 
right, and centripetal upon the left. The study of many sections, moreover, 
shows that other bundles lose protoplasm from the median tracheides first. 
Thus considerable irregularity of development of this tissue is established. 
Other phyla of living Pteridophytes show, as stated above, the same 
irregularity of xylem-development. Its occurrence, therefore, appears to be 
of no special importance in the Equisetales, the less so because a similar 
irregularity of development appears even in tracheides laid down by a 
cambium, as in the case of Betula cited above. 
The structure of the tracheary wall of these portions of the bundle was 
also studied. As the conditions of disappearance of protoplasm would 
cause one to suspect, there is some irregularity in structure ; reticulate and 
pitted tracheides occupy various positions in relation to scalariform elements 
in E. maximum and E. hyemale var. robustum . The examination of a 
considerable number of sections is clearly necessary, for, as Fig. 2 suggests, 
one may show apparently clear endarch, and another as clear exarch 
arrangement. In the majority of cases in these two species, however, 
development toward the outside was manifest. In E. arvense an unusual 
condition is found. The metaxylem-tracheides of some bundles are 
essentially alike, only very slight differences being noticeable ; those of 
other bundles are in part spiral and annular. Wherever this latter, not 
uncommon, condition occurs, the order of development is distinctly centri- 
fugal. A possibly parallel case has been noted in E. maximum by Queva. 1 
He figures (p. 16) a cross-section of a bundle in a long internode of the 
sterile stem. The metaxylem-groups are in part invaded by lacunae very 
similar in nature to that of the protoxylem. In the same investigation, 
Queva seems to have seen a rather regular development in this species. He 
says (p. 35), ‘ La differentiation est exclusivement centrifuge , mais le bois 
se forme en deux temps.’ His Fig. n shows this order. Thus, though 
lignification does not occur invariably in an outward direction, irregularities 
longitudinal shrinkage of their contents had occurred. Transverse sections were then cut so far 
down into the block as to be from a region the tracheides of which could not have been torn open 
and their contents in any way lost during the cutting-up of the material for killing and fixing. The 
infiltrating material, celloidin, was not removed from the sections, thus guarding against any loss of 
protoplasm by ‘ dropping-out.’ 
1 Queva, C. : Histogenese et structure du stipe et de la fronde des Equisetum. Mem. de la 
Soc. d’Hist. Nat. d’Autun, t. xx ; 1907. 
