XIV] ISOETES 65 



in an Isoetes stem. In the centre occur numerous spirally or 

 reticulate tracheae scattered in parenchymatous tissue which 

 has been considerably stretched and torn in the peripheral 

 region of the stele ; the radiating lines mark the position of the 

 leaf-traces (It) in the more horizontal part of their course. The 

 zone between the cambium (c) and the edge of the central 

 cylinder consists of radially disposed secondary tissue of short, 

 and for the most part unlignified, elements including sieve- 

 tubes and parenchyma; the secondary xylem elements consist 

 largely of thin-walled rectangular cells with delicate spiral 

 bands, but discontinuous rows of lignified tracheae (ccf) occur 

 in certain regions of the intracambial zone. The rest of the 

 stem consists of secondary cortex (cr) with patches of dead 

 tissue (6) still adhering to the irregularly furrowed surface. 

 The structure of the cambium and its products is shown in 

 the detailed drawing reproduced in fig. 133, D. Many of the 

 elements cut off on the inner side of the cambium exhibit the 

 characters of tracheids : most of these are unlignified, but others 

 have thicker and lignified walls (tr). 



I. hystrix appears to be exceptional in retaining its leaf- 

 bases, which form a complete protective investment and prevent 

 the exfoliation of dead cortex. Each leaf-trace consists of a few 

 spiral tracheids accompanied by narrow phloem elements directly 

 continuous with the secondary phloem of the intracambial zone. 

 Dr Scott and Mr Hill have pointed out that a normal cambium 

 is occasionally present in the stem of /. hystrix during the early 

 stages of growth; this gives rise to xylem internally: The 

 few phloem elements observed external to the cambium may be 

 regarded as primary phloem, a tissue not usually represented in 

 an Isoetes stem^ The occasional occurrence of this normal 

 cambium, may, as Scott and Hill suggest, be a survival from a 

 former condition in which the secondary thickening followed 

 a less peculiar course. The lower leaf-traces become more or 

 less obliterated as the result of the constant increase in thickness 

 of the broad zone of secondary tissues through which they pass. 



The adventitious roots are developed acropetally and 



^ Miss Stokey (09), in a paper which appeared since this account was written, 

 criticises the conclusions of Scott and Hill (00). 



