SECT. xiv. THE ROOTS. 409 



flower. Perpendicular plates, called medullary rays, 

 radiate from the medullary sheath and end at the bark, 

 dividing the whole mass of the wood into triangular 

 or wedge-shaped sections. They are thin plates of cel- 

 lular tissue, stretched horizontally between the central 

 pith and the bark. In each family of trees and shrubs 

 they have a different arrangement, but in all they keep 

 up a horizontal communication between the centre 

 and the circumference, though they do not all ex- 

 tend throughout the, whole length of the stem ; some do, 

 others do not. Thus the cellular tissue forms a hori- 

 zontal system, while the fibro-vascular ducts constitute a 

 perpendicular system of tissues. In some trees the pith 

 is scarcely perceptible, and in others it diminishes or 

 vanishes with age, as in the oak. In the alder and other 

 plants it dries up, breaks into pieces, and the canal is 

 filled with air. 



In the stem and branches of the Coniferse, there is 

 scarcely any mixture of vessels amongst the woody fibre, 

 the vascular system generally consisting exclusively of 

 glandular woody tissue, except in the medullary sheath, 

 where spiral vessels are found in small numbers. 



The subterranean growth, or descending axis of trees 

 consists of large branches, sometimes tending down- 

 wards, but more frequently spreading in extensive rami- 

 fications, not far from the surface of the earth. Their 

 growth and structure are similar to those of the stem, 

 but the cylinders of wood are less apparent ; they have 

 medullary rays, but no pith ; they merely connect the 

 active roots with the stem, and fix the plant firmly in 

 the ground, for they have few or no pores, and contri- 

 bute little to the nourishment of the plant, except by con- 

 veying liquids from the fibrous roots to the upper growth. 

 The active feeding roots spring from them in the form 

 of bunches of white fibres, like cords or threads, which 

 sink straight down into the ground. These real roots 



