146 



THE VASCULAR SYSTEM OF THE AXIS 



[CH. 



In many Ferns certain steps of the ontogenetic progression seen in these 

 examples are apt to be abbreviated, or even omitted: this is especially 

 the case where the internodes are short, as they are in upright stems. In 

 Acrostichmn aiireum, a Pterid-derivative which has an upright stock with 

 short internodes, the stages of the ontogeny are condensed, though on the 

 same plan as just described. The protostelic and the Lindsay a-s\.d.^^s are 

 brief, and the solenostely is established earlier. Such condensation is also 

 found in Paesia podophylla {Studies in Phytogeny, VII, p. 37), and in Pteri- 

 dium aqnilimim, according to Le Clerc du Sablon, and Jeffrey, and this 

 appears to be general for the Pterid-afifinity. It is this abbreviation which 

 probably accounts for the frequent failure to observe the simply medullated 

 state in the ontogeny of Ferns. It has been telescoped virtually out of 

 existence. Nevertheless it remains in such favourable cases as Gleichenia 

 pectinata and Loxsoma, and is there comparable with what is seen in the 

 adult state of most Ophioglossaceae and Osmundaceae. 



A special interest attaches to that intermediate state which has been 

 described by Tansley as the Li7idsaya-cor\da\\o\-\, because it has been re- 

 tained in that genus as the adult 

 structure, though in most Ferns 

 it is passed over quickly in the 

 course of transition from proto- 

 stely to solenostely. Sections 

 at an internode of Lindsaya 

 show an almost protostelic stele : 

 but an internal island of phloem 

 enclosed by outer xylem lies 

 near its upper surface in the 

 creeping rhizome, the whole 

 being enclosed in outer phloem 

 with unbroken outer endoder- 

 mis(Fig. 136). In theadult plant 

 the leaf-trace comes off as a 

 single sharply curved strand, 

 which on separating breaks the 

 dorsal arch of xylem covering 

 the internal phloem: thus the 

 latter is continuous with the phloem of the trace, which as it separates is 

 always limited externally by unbroken endodermis (Fig. 137). This condition 

 has been explained by supposing a very shallow pocket to be formed with 

 slight involution of the outer endodermis, but much deeper involution of 

 the outer phloem, so that this extends continuously through the axis. But 

 a better description would be in terms of the ontogeny, as revealed in 



Fig. 136. Transverse section of stele of Lindsaya linearis, 

 Swartz. G.-V. collection, slide 992. (X125.) 



