150 Boeshore — The Morphological Continuity of 



Further comparison of the last three genera will show that 

 the parasitic roots of Aphyllon do not cause truncation and de- 

 cay of the host roots, the latter remaining alive beyond the point 

 of attack, while in Epiphegus and Conopholis so complete is 

 the parasitism that host roots rarely remain alive beyond the 

 point of parasitic attachment. In almost all of the genera of 

 the Orobanchaceae parasitism has become highly specialized 

 with regard to selection of hosts. 



Thus a continuous and easy gradation is traced from green 

 and nearly independent Scrophulariaceae to highly condensed 

 and degraded Orobanchaceae, which in Conopholis closely simu- 

 lates the most degraded representatives of the Balanophoraceae 

 and Rafflesiaceae. On the other hand, no even approximate or 

 suggested such connection is shown between Gesneraceae and 

 Orobanchaceae. 



Histology of the Root 



In all the species of Gerardia examined the internal structure 

 of the roots is of the radial polyarch type. The xylem is great- 

 ly in excess of the phloem and consists in young roots, like those 

 of G. purpurea, mainly of large pitted vessels; spiral tracheae 

 and xylem cells make up the rest of the xylem. The phloem 

 arms alternate with the xylem and are composed of the usual 

 elements. In older roots of G. purpurea, G. flava, G. aphylla, 

 the bundle system assumes a woody more dense character, 

 having the appearance of a ring of wood not unlike that seen in 

 a dicotyledonous stem, save for the presence of the phloem. A 

 considerable amount of hard bast develops. A several-layered 

 pericambium surrounds the bundle region and this in turn is 

 surrounded by an endodermal layer, easily recognized in young 

 roots. The cortex is variously developed: in small, young roots 

 it consists of cells in rows, of 2 to 3 cells in each, and these are 

 radially disposed and connect the epidermis with the other tis- 

 sues, while between these radial arms are very large open spaces; 

 in older roots the cortical space is filled with normal thin-walled 

 cells; in more mature roots of G. flava the cortex is composed 

 almost wholly of scleroid cells staining a deep red color in saf- 

 ranin. In mature roots the epidermis may be replaced by cork 

 tissue. Root hairs were also noted. 



In the parasitism of Gerardia the roots produce swellings in 

 the regions of contact with the host, which become hemispher- 



