36 THE OAK 
ences of temperature (thermotropic), is not known, but 
it may be inferred that such is the case; nor do we 
know whether it is affected by electric currents in the 
earth. 
The root of the oak, speaking generally, is a typical 
root in the following respects. It consists, as we have 
seen, of a primary or tap-root which develops secondary 
or lateral roots in aeropetal succession, and these in their 
turn produce rootlets of a higher order. These second- 
ary, tertiary, &c., rootlets arise endogenously, taking 
origin from the pericycle at the periphery of the strand 
of vascular bundles which traverse the central axis, and 
then bursting through the cortex to the exterior. The 
primary root, as well as the rootlets of all orders, are 
provided with a root-cap at the tips, and they all agree 
in being devoid of chlorophyll or stomata. From the 
outer layer of cells—the piliferous layer, corresponding 
to an epidermis—root-hairs are developed at some little 
distance behind the root-cap, and these superficial cellu- 
lar outgrowths also arise in acropetal succession, the 
older ones behind dying off as the younger ones arise 
further forwards. If we bear in mind all that has been 
shortly stated above, it will be very easy to figure the 
behaviour of the rcot-sys‘em as it penetrates the ground, 
and the following short description of the biology of the 
root may render the matter clear. When the radicle 
commences to bore down into the soil it puts forth a 
large number of root-hairs from the parts a few milli- 
metres behind the tip, and these attach themselves to the 
