Tissues in Certain Monocotyledons. 51 
its inner side, and many layers of cork towards the exterior. 
The cells cut off by the phellogen often undergo further 
division by oblique walls, which may cause some disturbance 
of the normal radial arrangement. 
As the stem grows older, successive internal periderms 
are formed, until the whole of the primary cortex is cut off. 
This does not happen however until the outer zone of 
thickening has made good progress. Thus at the stage 
shown in Fig. 14, some primary cortex is still left. 
The formation of secondary cortex from the cambium 
does not begin until after the inner zone of wood 1 has 
been completed. During the development of the outer zone 
of wood, secondary cortex is formed with increasing rapidity 
(cf. Figs. 14 and 15 ; the latter was from the oldest part of 
the stem at our disposal). It may attain a thickness of about 
twenty layers before the primary cortex is lost. Ultimately 
the latter is cut off by periderm, and henceforth the entire 
cortex is secondary. The formation of internal periderms 
does not stop here however. The periderm shown in Fig. 15 
has evidently been formed in the secondary cortex itself. As 
new cortex is formed from the cambium, the older layers 
are constantly removed by more internal periderms. 
The periderm is provided with lenticels, but we did not 
follow their development. We may mention here that we 
found distinct indications of an abscission-layer at the base of 
the leaves. This subject also requires further investigation. 
3. Development of the Primary and Secondary Tissues . — 
It is clearly of importance to determine whether the 
formation of the secondary tissues is a mere continuation 
of the primary development (Sanio’s ‘ Thickening Ring ’ 
having an unlimited activity), or whether the cambium is an 
entirely secondary meristem, arising by the division of cells 
1 We use the term wood here for all the secondary tissue formed centrifugally 
on the inner side of the cambium. This is sanctioned by the authority of 
De Bary (Comparative Anatomy, Eng. ed. p. 591), but a better terminology is 
much needed. 
