58 THE MORPHOLOGY OF PTERIDOPHYTES 



plant indicates what the ancestral adult condition was like. 

 The stele of L. selago is similar to that of L. serratum, and 

 the number of radiating arms of xylem may be as low as 

 four. This is supposed to represent the ancestral condition 

 and the Selago subsection is regarded as primitive in its 

 stelar anatomy, as well as in its lack of a well defined strobilus. 

 Alternating with the xylem arms are regions of phloem, 

 separated from them by parenchyma, and the whole is sur- 

 rounded by parenchymatous 'pericycle', outside which is an 

 endodermis. The xylem strand of L. selago sometimes shows 

 a slight advance on this arrangement, in that it may be 

 separated into several areas with a variable number of 

 radiating arms. L. clavatumhsis a number of horizontal plates 

 of xylem, alternating with plates of phloem. An even greater 

 number of such plates is found in L. volubile (Fig. iiC). 

 To some extent this trend appears to be bound up with an 

 increasing dorsiventraUty of the shoot, which reaches its 

 culmination in the heterophyllous L. volubile. L. annotinum 

 lends some support to this idea, for its horizontal axes are 

 like those of L. clavatum, whereas its vertical axes are more 

 like those of L. selago. However, exceptions are numerous 

 and it may well be that no valid generalization of this kind 

 can be made.*^ 



Quite a different kind of complexity is illustrated by Ly co- 

 podium squarrosum (Fig. iiE), also placed in the Selago 

 subsection. A transverse section of the stem of this species 

 shows not only radiating arms of xylem, but also islands, 

 within the xylem, lined with parenchyma and containing 

 apparently isolated strands of phloem. Actually, however, 

 the whole structure is an anastomosing one, so that no 

 regions of phloem, or of xylem, are really isolated. This 

 process of elaboration has gone even further in L. cernuum, 

 where the appearance is of a sponge of xylem with phloem 

 and parenchyma filHng the holes (Fig. iiD). 



Throughout the genus, the stele is exarch, the proto- 

 xylem elements being clearly recognizable by their 'indirectly 



