40 PLANTS AND MAN 



must pass if they are to enter the vascular system. It is the most 

 important part of the root from the standpoint of absorption. 

 Outgrowths of the epidermal cells appear a short distance behind 

 the root-cap extending for an inch or two along the root surface. 

 These outgrowths, known as root hairs, (fig. 18) are slender 

 filaments which increase considerably the absorptive surface of 

 the epidermis and which are able to penetrate the soil in the 

 spaces between the soil particles. Root hairs are evanescent struc- 

 tures, living for only a day or two; new root hairs are produced in 

 the zone nearer the root tip as old ones wither away in the 

 region farther from the tip. Water passes into the protoplasm of 

 the root hair by osmosis, and at the same time any mineral salts in 

 solution enter the cell by dialysis. Once in the cell sap of the root 

 hair cells, the water and salts pass from cell to cell in the cortex 

 until finally these materials reach the central cylinder. Here are 

 two kinds of conductive channels; radiating from the center of the 

 root are groups of thick-walled water conducting cells into which 

 the water and minerals pass. The water conducting cells, or 

 XYLEM, form a continuous vascular system through the stem to 

 the veins of the leaves. Alternating with the xylem strands and 

 thus not interfering with the passage of water into them from the 

 root hairs, are groups of smaller food conducting cells known as 

 PHLOEM. The function of these vascular channels is conduction of 

 manufactured food from one part of the tree to another, bringing 

 soluble carbohydrates especially from the leaves to the stems and 

 roots. In addition to its conductive function, the central cylinder 

 can withstand the lengthwise tension to which roots are subject, 

 due to the thickened woody cell walls of the xylem. In older 

 roots certain changes occur in these structures; epidermis is 

 replaced by bark and a ring of growth cells produces annual 

 increase in thickness of the woody tissues. 



There is no sharp distinction internally between the root and 

 the stem. At the surface of the ground the root system becomes 

 the STEM, with little marked difference in appearance. A young 

 stem has epidermis but this is a short-lived tissue found com- 

 monly only in recent growth of the tree. In older stems and in the 

 trunk, the epidermis is replaced by bark as in the underground 

 parts of the tree (fig. 19). The cork cells which make up bark are a 



