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A TEXTBOOK OF THEORETICAL BOTANY 



Cortex 



Passage cell 



Endodermis 

 Protoxylem 



Pericycle 



Fig. 796. — Cypripediiim sp. Transverse section of root 

 showing O-tvpe endodermis with thin-walled passage 

 cell. 



The Stele. 



Inside the endodermis there is a pericycle consisting usually of a single 

 layer of thin- walled cells which retain their meristematic capacity for division, 

 both radially and longitudinally, even in the older root. It is from this layer 

 that the initials of lateral roots are formed and it contributes to the cambium 

 when secondary growth begins. 



The pericycle is differentiated very early at the root apex, and normally 

 from the same initial or initials as the rest of the central cylinder. The whole 

 mass of the central cylinder in its early state is to be regarded as procambium. 

 Pith is often absent, the xylem then developing to the centre of the root, but 

 when it is present it develops from the same common mass of tissue as the 

 vascular elements. Xylem and phloem arise in the central cylinder, the first 

 phloem elements preceding the xylem by a considerable distance. 



Although procambial elements are usually narrow and long, most Mono- 

 cotyledons and some Dicotyledons show, among the narrow elements, longi- 

 tudinal rows of large cells of squarish outline, which are the mother cells of 

 the large vessels. They appear quite close to the apex, but in spite of this 

 early differentiation they do not develop lignified walls, or lose their protoplasm 

 until after the protoxylem, which appears near the periphery of the plerome, 

 has completed its development. In transverse sections of young roots, these 

 large elements may be seen, still unlignified, lying near the centre of the 

 stele, at a stage when the outer xylem cells are completely matured. 



The first xylem elements to mature are those lying nearest to the peri- 

 cycle (Fig. 797) or, in the Grasses, in the pericycle itself. These are called 



