Scrophulariaceae and Orobanchaceae 153 



come semi-woody, semi-fibrous. With this degradation par- 

 asitism becomes of increasing importance. Further conden- 

 sation leads to Orobanche and Aphyllon with soft delicate roots 

 and such is accompanied by further simplification in root tis- 

 sues. From here the root system enlarges, becomes tuberous, 

 and is characterized by an increasing degree of hardness of the 

 tissues that reaches a climax in Conopholis. Further simplifi- 

 cation of tissues here results in unintelligible interpretation. 



Comparative Morphology and Physiology of Stem and 



Leaf 



Under this heading stems and leaves will be considered. The 

 drawings accompanying this part of the paper were made to il- 

 lustrate more particularly relative lengths of stems. 



In the most primitive Gerardias the stem is tall, well formed, 

 and typically normal dicotyledonous, attaining a height of 1 to 

 4 feet in G. flava (Plate XIV, Fig. 9). The lower leaves are 

 large, ovate-lanceolate, sinuate-toothed along the margins; in the 

 upper leaves the margins are entire. Both stem and leaves are 

 covered with a fine close down. 



Transverse sections of the stem show a large pith area, a ring 

 of xylem with numerous pitted vessels and internal to these spir- 

 al tracheae, a cambium, a narrow zone of phloem consisting of 

 little soft and much hard bast. Externally the phloem is bound- 

 ed by a cortex zone of 1 to 5 layers of cells, with the outer region 

 collenchymatous and the inner region of thin-walled cells. The 

 epidermis is persistent around the stem and from it project the 

 numerous two to four-celled pointed hairs. The basal cell of 

 these is large, rounded; the outer cells are narrow with the tip 

 cell pointed. 



G. purpurea, with stems 1 to 3 feet tall, has a comparatively 

 small pith, but a wide zone of xylem. The phloem consists 

 mainly of soft bast with occasional patches of hard bast. Scler- 

 enchyma cells are a frequent feature of the cortex. The leaves 

 of this species are much reduced in size, linear, acute, rough- 

 margined according to Gray (21, p. 731). The rough character 

 is due to many one-celled, pointed, more or less spiny hairs distri- 

 buted over the general leaf surface and especially along the leaf 

 margins. 



