VIIl] 



POLYCYCLY 



151 



simple solenostele complicated by corrugation, by self-fusions, and by per- 

 foration. In the internodes the corrugation is often continued, with many 

 deep folds (Fig. 143). The effect is greatly to enlarge the surfaces of the 

 thin expanded solenostele, which will naturally aid gaseous and other inter- 

 change. But since the endodermis is a closed sheath the effect is less than 

 that of perforation. It may be compared, as regards method, with what is 

 seen in the stellate protostele of AsterocJilaena: in both cases the corrugation 

 appears in 'organisms of considerable size (see Chapter x). 



Fig. 142. Transverse section of soleno- 

 slelic rhizome of Cibotitiiii {Dkksotiia) 

 Barometz. Natural size. 



Fig. 143. Hisiioptensiniisa^\\Mwhg.),'].^\-iK. Trans- 

 verse section of internode of rhizome ( x lo) show- 

 ing corrugation of the solenostele. (Gwynne-Vaughan 

 collection, slide 1163, by Tansley.) 



POLYCYCLY 



In other groups of plants, such as the Cycads and Angiosperms, large 

 parenchymatous masses are commonly traversed by accessory vascular 

 tracts. This is found also in Ferns, giving rise to those states described 

 under the general term of "polycycly," since in conformity with the vascular 

 evolution of Ferns these inner conducting tracts are arranged in more or 

 less regular cycles, one within another. Examples are seen in the Matonia- 

 ceae, Cyatheaceae, and Marattiaceae among the Superficiales, and in the 

 Dicksonieae and Pterideae among the Marginales. The occurrence is so 

 sporadic that it can only have been initiated along a plurality of evolu- 

 tionary lines. As Tansley says, " it is no doubt a response to a need for an 

 increased vascular supply." The need for ready conduction along the axis, 

 as well as to and from the enlarged pith, appears a sufficient explanation of 

 the occurrence of polycycly. 



Gwynne-Vaughan first showed the gradual steps which led to polycycly 

 in the Pterideae. In Dennstaedtia apiifolia there is a typical solenostele- 

 but there is a thickening of the xylem at the margin of the leaf-gap. In 

 D. adiantoides this thickening projects into the pith, and continues as a 



