82 



BOTANY OF THE LIVING PLANT 



primary wood. If the protoxylem is central the section has come 

 from a stem ; if it is peripheral relatively to the rest of the primary 

 wood, then the section has come from the primary region of the root. 



As the distal part of a young root grows older the root-hairs upon 

 it shrivel. In Monocotyledons, and in some Dicotyledons the cortex 

 is retained ; but as it grows old its cells lose their turgor, and the 

 cortex shrinks. In bulbous Monocotyledons, and in some Dicoty- 

 ledons the whole root then shortens, anchoring the plant firmly in 



pr.plil- "'- 



FIG. 64. 



Diagram A shows arrangement of tissues in a young root of a Dicotyledon before 

 cambial activity begins. B, the same when cambium can be clearly recognised. 

 C, after secondary thickening has progressed. c=cortex, present in A and B, but 

 in C it has been thrown off. pr.phl= primary phloem. phl"= secondary phloem. 

 pr.xy= primary xylem. xy"= secondary xylem. p.mr= primary medullary ray. 

 mr"= secondary medullary ray. w=pith. F. O. B. 



the ground, while evidence of the shrinkage is seen in transverse 

 wrinkles on its surface. 'In most woody Dicotyledons the bulky 

 cortex itself collapses and finally peels off. This is due to the forma- 

 tion of a band of cork, which originates from the pericycle, and cuts 

 off the outer-lying tissues from physiological connection with those 

 within, so that they perish. Since the cortex makes up a very large 

 proportion of the whole bulk of the root, the consequence in such 

 cases is that at first the root appears to become thinner as it grows 

 older. But as a matter of fact the central stele has meanwhile been 

 increasing in bulk, and it is protected externally by the band of cork 

 which originates from the pericycle. Since this cork may be further 



