Scrophulariaceae. 459 



soon as cork-formation commences, yet how early in the 

 development of the runner I am not prepared to state, but 

 it probably happens rather early. The amount of cork 

 developed is small; more than five layers of corky cells (the 

 thin- walled elements placed in radial rows seen in the figure) 

 never occur, in fact, often only 2 — 3 layers; sometimes the 

 cortical layer situated immediately under the epidermis, 

 forms the cork-cambium. This cork-formation is continued 

 from the runner-hke,into the rosette-bearing part of the shoot. 



In the stele, which is surrounded by an endodermis of 

 a similar kind as that of the root (in Fig. 35, В the inner- 

 most circle indicates the endodermis), two groups of xylem are 

 found; the vessels are intermingled vàlh. non-lignified paren- 

 chyma, and the groups are separated by two broad medul- 

 lary rays, diametrically opposite to each other, in the outer 

 part of which a small group of narrow vessels occurs (leaf- 

 traces). The pericycle is several-layered; like the cortex, it 

 consists of cells with somewhat thickened walls. In the peri- 

 fery of the xylem-groups a few wood-fibres sometimes occur. 



The Leaf. The epidermis of the upper surface has 

 lateral walls varying from straight to slightly undulating, 

 comparatively thick, but only slightly poröse; on the lower 

 surface of the leaf the lateral walls of the epidermal cells 

 are found to be shghtly undulating; the cells are essentially 

 more thin-walled than those of the upper surface. Stomata 

 occur only on the lower surface of the leaf; they are sur- 

 rounded by 4 — 9 cells. 



A transverse section of the leaf is shown in Fig. 35, E\ 

 as far as I could judge from the material at my disposal, 

 the portion which has been selected for illustration is typical : 

 2 layers of palisade-cells, and a few-layered spongy paren- 

 chyma with cells, which do not, on the whole, branch very 

 copiously, are seen. 



