28/ 
and Histogeny of Strychnos. 
the centripetally active cambium, the growing tissue has to 
find room among the pre-existing bast-parenchyma, which 
consequently becomes locally compressed. At this stage the 
transverse section of the developing island has the form of a 
segment of a circle (the cambium forming the chord), which 
projects into, and displaces the bast-parenchyma. Among 
the crushed elements bordering the new tissue one of the 
primary phloem-groups may occur, but this is not constantly 
the case. The cells cut off by the cambium rapidly pass 
over into permanent tissue. Each sieve-tube is formed 
directly from a cambial cell, the only divisions which it under- 
goes being those by which its companion cells are formed (see 
Figs. 6 and 8). 
The space required by the developing phloem-island is 
however only partly provided by displacement of the external 
tissue. The cambium forming the island soon ceases, or 
nearly ceases to produce wood, and its activity in phloem- 
formation now just balances the activity of the normally wood- 
forming cambium on either side. Hence the new phloem 
comes to be flanked on the right and left by newly formed 
secondary xylem, and thus lies in a depression of the woody 
mass (see Fig. 7). That this process must be accompanied 
by £ sliding growth’ between the phloem and the adjacent 
wood is evident here, as it is in the case of the phloem-plates 
of Bignonia , with which the islands have already been com- 
pared by Herail h Hence we find that the parenchymatous 
cells lying on the flanks of the island become stretched in the 
radial direction. The unequal formation of xylem and phloem 
further results in the fact that the phloem-forming portion 
of the cambium becomes detached from the normal ring, 
and remains nearer the centre of the stem. Thus the main 
zone of cambium has now suffered an interruption of its 
continuity. But this interruption is only temporary. The 
process just described goes on until the phloem-island has 
1 For a very full account of the development of the phloem-plates in Bignonia , 
see Hovelacque, Recherches sur l’appareil vegetatif des Bignoniacees, etc., 1888, 
pp. 60-69. 
