and Histogeny of Strychnos . 
303 
EXPLANATION OF FIGURES IN PLATES 
XVIII AND XIX. 
Illustrating Messrs. Scott and Brebner’s paper on Strychnos. 
The following lettering is used throughout : — 
cb l = Normal (including complementary) cambium. 
cl) 1 — Medullary cambium, adding new elements to the medullary phloem- 
groups. 
cb z = Cambium from which the phloem-islands are produced. 
ph v — Normal external (rudimentary) phloem. 
ph? — Medullary phloem. 
ph % — Phloem-islands, ultimately imbedded in the wood. 
x l -- Primary xylem. 
x 2 = Secondary xylem. 
pc = Pericycle. 
p — Pith. 
Ip = Lignifiea pith. 
r = Medullary ray. 
c = Companion cell. 
s = Sieve-tube. 
Fig. 1. A. spinosa, Harv. x 500, transverse section. Young vascular bundle, 
showing external phloem-groups, and the medullary group in course of 
formation. 
Fig. 2. S. spinosa, x 920, transverse section. External phloem-groups at a 
slightly older stage. The sieve-tubes (with sieve-plates) and companion cells are 
shown. Only young unlignified xylem is shown. 
Fig- 3- S. spinosa, X920, longitudinal section. At the same stage as the last, 
showing a sieve-tube and companion cell in an external phloem-group. A nucleus 
is present in the sieve-tube. 
Fig. 4. S. nux vomica, L., x 333, transverse section. Shows a medullary 
phloem-group. On its outer side is an active cambium, while on the inner side 
the elements are beginning to become obliterated. 
Fig. 5. S. nux vomica, x 333, transverse section. More advanced medullary 
phloem-group. The cambium is ceasing its activity, and a great mass of obliterated 
elements is seen on the inner side. 
Fig. 6. S. nux vomica, x 333, transverse section. First commencement of the 
formation of a phloem island. The normal cambium is increasing its activity in 
this region, and has already produced a few sieve-tubes and companion cells. 
The compressed elements may be primary phloem. No retardation of wood- 
production is as yet to be noticed. This is from an old stem (thickness 5 mm.). 
