ROOTS: KINDS OF ROOTS 



37 



nating with these four radiating groups of wood cells are four 

 other groups of bast cells (or phloem), which lie near the peri cycle. 

 The tissue of thin-walled cells lying between these groups is 

 fundamental tissue, parenchyma (paragraph 100). 



61. Origin of the tissues of the root. This can be studied 

 in a longitudinal section of a young root including the tip. The 

 epidermis is the outer 

 layer of cells (0, fig. 

 40) . Towards the root 

 tip the rows of epi- 

 dermal cells converge 

 on a curve and meet 

 near the middle in a 

 tissue of cells rich in 

 protoplasm and with 

 thin walls (the meri- 

 stem, a group of grow- 

 ing cells at the end of 

 roots and stems). The 

 cortex (p', fig. 40) lies 

 just inside of the epi- 

 dermis and consists of 

 several rows of cells on 

 each side. Inside of 

 the cortex is the central 

 cylinder (p, fig. 40). 

 The meristem is the 

 tissue from which the 

 others arise. Its cells 

 divide and the older ones pass back into epidermis, cortex 

 and central cylinder. In the central cylinder there is a great 

 variety of cells. In the meristem where these cells arise they 

 are all alike, but as they age and pass over into the different 

 tissues they take on the special forms which make them suitable 

 for their special kind of work. This is an interesting and im- 

 portant change, since economy and the highest utility are gained 



Fig. 40. 



Lengthwise section (somewhat diagrammatic) through 

 root tip of Indian corn, w, root cap; !, younger part of 

 cap; z, dead cells separating from cap; s, growing point; 

 o, epidermis; p', intermediate layer between epidermis 

 and central cylinder; p, central cylinder, in which the 

 fibro-vascular bundles arise. After Wiesner. 



