68 
When one of the largest hairs of the three species Is grasped by 
forceps it breaks off at the base. Upon contact with an object the hollow 
heads at the apex are also broken off. In the case of the smaller hairs 
because of the fineness and brittleness of the portion near the apex the 
point is broken off when contact with a resistant substance is mode. \n 
each instance the apical portion of the hairs is left with a fine, jagged, 
often oblique, hollow point which allows the contents of the hairs to 
emerge. These observations explain how the hairs on contact or pressure 
are forced into the skin and become detached from the plant. 
The hairs of all types can be compared with hollow needles. The 
larger hairs are in reality giant cells. The cell contents of the larger 
hairs when viewed microscopically are variously coloured greenish yellow, 
brown, and in parts opaque or black, and except for the darker portions 
they are more or less transparent. Their texture varies from apparently 
structureless to finely particulate. The particles vary in size from 4 
microns to less than 1 micron In diameter; they often appear to have a 
dark periphery and a pellucid interior, appearances which are familiar 
to microscopists as apparently similar to those presented by oil particles. 
Chloroplasts were observed in some of the hairs of the Gympie and the 
giant stinging tree. In a few of the hairs of these two species rectangular. 
Isotropic crystals were observed. 
Nuclei 30-45 microns in diameter were seen in some of the larger 
hairs of the three species. In some cases it was noticed that the nuclei 
were os large or even larger than the epidermal ceils near the base of 
the hairs. 
On the outside and surrounding the base or slightly above it In the 
larger stinging hairs is an elegant ring or 'collorette' of flattened, apically- 
rounded, epidermal cells. This ring is the apical portion of the pedestal. 
See figs. 5, 6 and 7. All of the hairs in the figures are from the upper 
surface of the leaves. When viewed in polarised light the walls of the 
hairs are birefringent. An unusual feature of the heads of the hairs of 
the giant stinging tree and the Gympie is that they often show a dark 
cross between crossed Nicol prism.s. The cross is similar to that showrt 
by starch grains. 
The Active Principle of the Stinging Sensation 
The problem of the active principle of the sting of the commort 
nettles (Urtica spp.) has been a subject of investigation for a very long 
time. Solereder (5) states that ''Since the work of Gorup-Besanez the 
irritant causing the stinging sensation has usually been stated to be 
formic acid; this, however, Is incorrect, and it is probably a substance 
related to the ferments". Flury (2) found that the nettle effect is produced 
by a non-volatile, unsaturated, nitrogen-free compound of acid nature, 
which, according to its properties, closely approaches the resin acids. The 
stinging-nettle poison Is neither formic acid, nor an enzyme nor a toxal- 
bumin. Incidentally I may mention that Flury in the introduction to his 
paper states that there are many indications of a close homogeneity of the 
poisons generated by marine nettle animals and the plant nettle poisons. 
The principal species investigated by Flury was Urtica dioica, the common 
nettle. Emmelin and Feldberg (1) attribute the sting of the stinging nettle 
to histamine and acetylcholine. The species they investigated was Urtica 
urens. 
The stinging heirs of the giant stinging tree were investigated 
chemically by J. M. Petrie (4) who concluded that their stinging property* 
Is due to the free concentrated formic and acetic acids contained in them. 
He found the hairs to be composed of silica. The stinging hairs of the 
Gympie and the shining-leaved stinging tree do not appear to have been 
investigated. 
