ANATOMY 419 



protoxylem makes its appearance : where the protoxylem is clear it is 

 evident that the differentiation of the primary xylem is centripetal, as it 

 is, with local exceptions, throughout the aerial shoot. Passing upwards 

 along the aerial shoot, the peripherally projecting protoxylems increase 

 in number, the whole stele enlarging : finally, in transverse section the 

 xylem appears as a hollow, many-rayed star, while the centre is occupied 

 by sclerotic tissue. Peripherally as before lie the phloem and the endo- 

 dermis (Fig. 233). An examination of the lower part of the aerial shoot, 

 and of some adjoining parts of the rhizome, shows a feeble secondary 

 formation of xylem : there is no definite cambium, but the additional 

 tracheides which arise from the tissue outside the primary wood often 

 show signs of a radial arrangement. The secondary development fades 

 off as the stellate structure of the upper region is attained. Thus 



FIG. 233. 

 Psitotjiiu triquetrum. Part of a transverse section of the central stele. X 100. 



structurally the base of the stem of Psilotiim recalls the stem of Spheno- 

 phyllitm, though with very feeble secondary growth, while the upper part 

 of the stem resembles the axis of the cone of Cheirostrobus, though on a 

 simple scale, and with fewer xylem-rays. 



In the stem of Tmcsipicris no secondary development has hitherto 

 been found. The rhizome exhibits much the same structure as that of 

 Psilotiim, with a solid group of tracheides at the centre, or in weaker 

 branches an irregular xylem-ring, surrounded by phloem, and a very ill- 

 defined endodermis. Often there is no distinct protoxylem ; but, as the 

 passage is made to the aerial stem, protoxylem may appear : its position 

 in all the upper region is mesarch. In weaker shoots, and especially in 

 the upper regions, the cauline tissue of the stele fades out : the leaf- 

 traces become the main feature of the vascular system, which in trans- 

 verse sections is then represented by a ring of separate strands : each 

 of these has its mesarch protoxylem corresponding to that seen in the 

 leaf-trace (Fig. 234): in point of fact this becomes a phyllosiphonic type 

 of structure. In strong axes, however, the xylern may still form a 



