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A TEXTBOOK OF THEORETICAL BOTANY 



Anatomy of the Root 



The adventitious roots arise at the bases of the leaves, though their steles 

 are in direct communication with the stele of the stem. The development 

 of these roots begins in specialized cells of the endodermis of the stem stele. 



In structure the root is relatively simple (Fig. 476). In the centre is a 

 small xylem with tw'o protoxylem groups, one at each end. A stele like this, 

 in which there are two protoxylem groups, is said to be diarch. The xylem 

 plate between the protoxylems has a constant orientation parallel to the axis 

 on which the root is borne. Outside the xylem lie two groups of phloem 

 elements arranged laterally to the xylem plate. There is also a limited 



Protoxylem 



Fig. 476. — Dryopteris filix-ynas. Transverse 

 section of root. 



parenchymatous conjunctive tissue. Surrounding the phloem is a double- 

 layered pericyle, followed by a single-layered endodermis. The cortex 

 consists of a vtry thick-walled inner zone and a thin-walled outer zone, so 

 that it appears double. Like the stem, the root has a single large, pyramidal, 

 apical cell with three sides, from which all cells of the organ, with the 

 exception of the root cap, are cut off by successive divisions. The cells of 

 the root cap are cut off from the base of the apical cell. Each cycle of three 

 divisions parallel to the sides of the pyramid is followed by one parallel to 

 the base, so that every fourth cell formed is added to the tissue of the root cap. 



Development of the Sporangium 



The spores are produced on certain leaves of the mature plant which 

 may therefore be called sporophylls. In Dryopteris the vegetative leaves 



