Fes, 1909.]| | BOTANICAL SOCIETY OF EDINBURGH 25 
show that as the sections approach the tip of the thorn 
the wood tissue is considerably increased and the aeren- 
chyma gradually lost (figs. 6, 7,8). A section near the tip 
shows practically all hardened tissue except phloem. As 
Dr. Friedrich puts it, with the whole change of the aerial 
root into thorns nothing is altered in the anatomical 
arrangement of the inside tissues; the cells of all the 
tissues, with the exception of the phloem become thickened 
and lignified. The flattened bases of some of these aerial 
roots (fig. 9) exhibit the same essential features as the 
circular roots themselves, but with a much more marked 
woody development. 
Mention was made of the roots which are found, on 
tearing down the leaf-sheath, flattening themselves against 
the trunk. The point is interesting, for they resemble in 
this respect the intracortical roots of the Bromeliacez ; for 
example in Puya and Kingia. These roots, one will re- 
member, are developed inside the cortex, and on emerging 
lose their root-cap, a further point of resemblance to a stage 
in the development of the aerial roots in Acanthorhiza. 
The flattened roots may be simply explained as the pressure 
of the leaf-sheaths, since on emerging they become cylindrical 
in form; but the striking resemblance in habit to the intra- 
cortical roots of the Bromeliads, and the tendency of the 
roots to lie parallel with the stem, suggests another cause, 
that of atavism, that is to say, that the memory of a former 
habit similar to the roots of the Bromeliads still clings to 
them. 
The interesting point to notice about these aerial roots 
is their function; there is no doubt that in their early 
existence they function as pneumathodes or breathing 
roots, an inference drawn from the presence of much 
aerenchyma. This function persists even in the thorn 
stage, where aerenchyma are found at the base of the 
structures, but with the loss of the root-cap and aeren- 
chyma the function becomes largely that of protection. 
The fact that many of these roots do not proceed as 
far as the thorn stage, but on entering the ground 
develop as normal roots, adds a third function to those 
already mentioned—one similar to that of the prop-roots 
of the Pandanacez. 
