86 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 acropetal 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 



