ROOTS AND THEIR FUNCTIONS 145 



tion in the walls of the epidermal cells of a corky substance called suberin 

 (from Quercus suber, the "cork oak") — the same material that makes 

 cork so impenetrable to water. 



The structure of a young root. The arrangement of tissues in the 

 root is best seen in a cross section through the region of root hairs, where 

 the cells produced by the growing point have become differentiated into 

 tissue cells but where complications due to secondary thickening have 

 not yet entered. At the surface is the epidermis, with its root hairs. 

 Beneath the epidermis is a zone called the cortex. Most of the cortex 

 is composed of a spongy tissue called parenchyma, made up of rounded 

 cells with thin walls and large vacuoles; the cells of the innermost layer 

 of cortex have thickened walls, and this layer, which surrounds the stele, 

 is called the endodermis. 



The inner core of the root, including everything inside the cortex, 

 comprises the stele. l Its outermost layer is a cylinder of parenchyma cells 

 called the pericycle, which gives rise to branch roots and which in peren- 

 nial plants plays a part in the radial growth of the root. Beneath the 

 pericycle, the stele contains the vascular system, the cambium layer, 

 when one is present, and sometimes a central pith and groups of fibers 

 that add to the strength of the root. 



The vascular system consists of two specialized types of tissue which 

 together form the transporting or conductive system of the plant. One 

 of these, the xylem, is essentially a structure of thick-walled dead cells, 

 so arranged and connected as to function as water conduits. The other 

 tissue, the phloem, is composed of living cells connected through openings 

 in the cell walls. In the phloem, substances in solution pass through the 

 protoplasm from cell to cell. The structure and functions of these vascular 

 tissues are more fully treated in dealing with the stem. In a cross section 

 of a young dicotyledonous root the primary xylem forms a star-shaped 

 figure with three, four, or five rays; it generally extends to the center 

 of the root. The strands of primary phloem lie between the rays of the 

 xylem star, separated from the xylem by parenchyma; in cross section, 

 they appear as small, isolated cell groups. 



The structure of older roots. In many plants, including most annuals, 

 the roots do not increase in thickness. In such plants the structure of the 

 older parts of the root does not differ in any essential respect from that 

 just described, and such roots lack a cambium layer. In perennial plants, 

 on the other hand, the year-by-year growth of the whole plant makes 

 necessary additional support by the roots and an increase in the capacity 

 of the vascular system. The roots of such plants grow in diameter with age. 



This is made possible by the presence of the meristematic layer called 



1 Pronounced sWU. 



