I08 THE MORPHOLOGY OF PTERIDOPHYTES 



The sporangia are borne in a cone, which in some species 

 {Equisetum arvense) terminates a special aerial axis that 

 lacks chlorophyll, is unbranched and appears before the 

 photosynthetic axes. In other species the fertile shoot is 

 green and may subsequently give rise to vegetative branches 

 lower down (e.g. E. limosum and E. pratense), after the cone 

 has withered. In yet other species, most of the lateral 

 branches may terminate in a cone (e.g. the Mexican species 

 E. myriochaetum). This last arrangement is commonly re- 

 garded as the primitive condition, on the basis that it in- 

 volves the least speciahzation, but it must be reahzed that 

 real evidence for this view is lacking. 



The internal anatomy of the stem of Equisetum presents 

 an interesting association of xeromorphic and hydromorphic 

 characters, together with a vascular system which is without 

 parallel in the plant kingdom today, and whose correct 

 morphological interpretation has long been the subject of 

 controversy. The ridges in the stem are composed of 

 sclerenchymatous cells, whose thick walls are so heavily 

 sihcified as to blunt the edge of the razor when cutting 

 sections. Stomata are restricted to the 'valleys' between the 

 ridges and are deeply sunken into pits whose openings may 

 be partly covered by a flange of cuticle. The walls separating 

 the guard cells from their accessory cells bear pecuHar comb- 

 like thickenings which are known elsewhere only in the 

 leaves of Calamites. Beneath each of the valleys is a 'vallecu- 

 lar canal' and the central region of the internodes of aerial 

 stems consists of a large space (but, in subterranean stems, 

 the centre may be occupied by pith). At the nodes, there is a 

 transverse diaphragm. Such an arrangement of air channels, 

 together with a very reduced vascular tissue, are features 

 normally found in water plants and contrast strikingly with 

 the heavy cuticle, sunken stomata and reduced leaves. 



The internodal vascular bundles lie beneath the ridges of 

 the stem and are quite characteristic (Fig. i6Q). As in 

 Calamites, the protoxylem is endarch and is replaced by a 



