Tissues in Certain Monocotyledons. 55 
formation has again fairly commenced, and the secondary 
period of growth is entered upon. 
A stage similar to this (from another series) is shown in 
Fig. 13. At least two of the original pericyclic cells may 
contribute by their divisions to form one secondary bundle. 
At the end of our series, at 52 mm. from the apex, the 
secondary zone is slightly more advanced. 
The whole series is most instructive, showing that there is 
a long interval between the cessation of the primary develop- 
ment and the commencement of secondary increase. At 
5 mm. from the apex the primary merismatic divisions had 
almost ceased ; only at 31 mm. had anything approaching to 
a continuous cambium arisen by fresh divisions, and the first 
formation of secondary tissues did not begin until a distance 
of 47 mm. from the apex was reached. There is thus a per- 
fectly definite distinction between the primary and secondary 
tissues, though individual vascular bundles may be common 
to both, as indeed is necessary in order to keep up physio- 
logical continuity. 
In feeble branches the interval between primary and 
secondary development is much less marked, and may even 
be almost obliterated. This is evidently due to a ‘ telescop- 
ing’ of the developmental stages, and does not affect the 
conclusions drawn from vigorous shoots, to which we must 
look for the typical mode of growth. 
We will now complete, from another series, our account of 
the secondary development. 
The mode of formation of the inner zone of thickening is 
peculiar. No regular radial series can be traced, and in fact 
there is no single continuously active layer of cambium. A 
cell of the pericycle divides up a few times — say six — by 
tangential walls formed in centrifugal order; the daughter-cells 
subdivide to form the elements of a bundle, or may directly 
become cells of the secondary ground-tissue. Meanwhile 
another pericyclic cell, on the distal side of the first, has 
begun to divide ; this contributes its share, and then in turn 
its activity ceases, and so on. Hence the fully formed 
