LYCOPSIDA 77 



section through an old plant of Isoetes. To the right and left 

 are the shrivelled remains of the previous year's growth, the 

 several sporophyll-traces and root-traces being visible 

 within it. All the rest represents the present year's growth 

 surrounding the perennial central regions. Occupying the 

 centre is a solid protostele, the lower part of which is 

 extended into two upwardly curving arms, so that the over- 

 all shape resembles an anchor (Fig. 13G). This is made up of 

 mixed parenchyma and pecuUar iso-diametric tracheids 

 with hehcal thickenings. Towards the outside the tracheids 

 are arranged in radial rows but, nevertheless, they are of 

 primary origin (indeed, some workers hold that the whole 

 of the primary wood is protoxylem). Surrounding this 

 primary wood is a narrow zone of phloem (not shown in 

 Fig. 13F), and outside this is the tissue produced centri- 

 petally by the anomalous cambium. This commonly consists 

 of a mixture of xylem, phloem and parenchyma and is des- 

 cribed by the non-committal term 'prismatic tissue'. The 

 cambium, represented in Fig. 13F as a broken line, cuts 

 through the sporophyll-traces and root-traces of previous 

 years, leaving their truncated stumps still in contact with 

 the primary wood. 



What little vertical growth there is takes place by means 

 of apical meristems at the top and bottom of the corm. The 

 lower of these is extended as a line beneath the anchor- 

 shaped primary xylem and is buried deeply in a groove. 

 Roots arise endogenously along the sides of this groove in a 

 very regular sequence and are carried round on to the under- 

 sides of the newly formed cortex. The stem apex is also 

 deeply sunken between the 'shoulders' of the corm and is 

 said to contain a group of apical initial cells. Sporophylls 

 arise in spiral sequence (with a phyllotaxy of |, t\ or 

 2®i in mature plants) and, as new secondary cortex is 

 formed, they are carried up on to the shoulders. 



Stages in the development of the young sporophylls are 

 illustrated in Figs. 13J-L. At a very early stage, when the 



