288 Scott and Brebner. — On the Anatomy 
attained its definitive form. Then the activity of the phloem- 
forming cambium slackens, though it does not cease, and so 
the margin of the wood comes up flush with the outer margin 
of the phloem-island. And now those cells of the external 
bast-parenchyma which have escaped obliteration begin to 
divide, starting on either side with the cells adjacent to the 
normal cambial zone. The divisions advance from both sides 
until the cambium becomes continuous around the outer 
border of the island, and the normal ring is once more com- 
pleted (Fig. 9). 
The exact position of the cells which divide, in order to 
complete the cambial zone, varies. They may either lie 
immediately outside the external sieve-tubes of the island, 
separated from them only by a layer of obliterated cells ; or 
they may lie further towards the exterior, leaving one or two 
rows of living parenchyma between the new cambium and the 
outer border of the actual phloem. The cambial arc by which 
the normal ring is completed requires a name. We propose 
in this, and in all similar cases, to term it complementary 
cambium. A word must be said as to the nature of the cells 
by the divisions of which it is formed. Herail speaks of them 
as belonging to the pericycle. This may be true in some 
species, in which the pericycle remains largely parenchy- 
matous, but in .S'. nux vomica the inner cells of the pericycle 
become sclerotic, and the cells which undergo division, as 
shown by their position and regular radial arrangement, both 
in transverse and longitudinal sections, are clearly of secondary 
origin, and come under the head of bast-parenchyma. 
The complementary cambium now assumes its normal func- 
tion as part of the general ring. It begins to form wood, 
outside the new phloem, starting on either side (Fig. 10). 
The wood thus formed becomes continuous, and the 
phloem-island is now completely surrounded, and deserves 
its name. 
The cambium on the inner side of the island does not how- 
ever cease its activity. It goes on for a long time producing 
new and active phloem as the old becomes past work, and the 
