36 6 Clark . — Secondary Thickening in 
Unguis there is a ‘breaking up of the xylem into separate masses, often 
by a certain amount of dilatation-parenchyma’. Unfortunately he does not 
state the origin of the dilatation-parenchyma, nor the mode of breaking up 
of the xylem, so that a further comparison of this root with the Kendrickia 
stem cannot be made. 
In the stems described above, the splitting which takes place is not, 
strictly speaking, an entirely new formation. The medullary rays con- 
tained in the axial woody ring connect the furrows with the pith, and 
therefore from the first actually map out the wood by fine lines of divisions 
into its various segments. Thus by the dilatation and new growth of 
apparently mature parenchyma a microscopic line of separation is con- 
verted into a large conspicuous split. 
In Keiidrickia the axial woody ring contains no medullary rays, there- 
fore there are no existing lines of separation of which to make use. Yet 
in both cases, though by widely different methods, the same result is 
reached, viz. large parenchymatous communications between the internal 
and the external soft tissues. 
Schenck 1 discusses the causation of this new growth, and advances as 
probable the theory that the continued renewal of the bast-plates causes 
a tangential or radial tension in the axial woody ring, which acts as 
a stimulus, leading to a dilatation of the lines of least resistance, viz. the 
primary medullary rays and the pith. In Kendrickia, however, the first 
evidence of the change about to occur is the dilatation and division of one 
or more cells of the wedge, therefore there is nothing in the nature of 
preliminary anatomical changes which could constitute a stimulus, tan- 
gential or otherwise. 
Farther, in the stems mentioned above, the woody ring is divided 
into a definite number of segments, each with a distinct portion of pith 
attached to its inner margin ; and immediately succeeding the splitting, 
isolated secondary cambiums become evident at varying intervals in the 
dilatation-parenchyma. In Kendrickia , on the other hand, the separated 
portions of wood are so small in many cases that they by no means 
deserve the name of segments ; also the pith, if attached, is not always 
recognizable as such ; and lastly, isolated, secondary cambiums are never 
developed in the dilatation-parenchyma, even in stems at the age of that 
in Fig. 17. It is quite possible, however, that they may develop in still 
more mature stems. 
Thus it is seen that, although the splitting of the axial woody ring is 
by no means a rare phenomenon in plants, yet the details of the process in 
Kendrickia are in some ways quite unique. The part which tylosis plays 
in the process, by utilizing the space contained in the lumen of the tracheids 
and vessels, with the subsequent destruction of the identity of these cells, 
1 Loc. cit., p. 231. 
