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A TEXTBOOK OF THEORETICAL BOTANY 



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, Piliferous layer 





Metaxylem ■•#»' 





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Fig. 799. — Vtcia faba. Transverse section of a root 

 showing the endogenous origin of a lateral root 

 from. the pericycle opposite a protoxylem group. 





Secondary Thickening. 



When the differentiation of the primary metaxylem is complete, or nearly 

 complete, a cambium makes its appearance, in roots which undergo 

 thickening. This includes most roots except those of Monocotyledons 

 and those of short-lived annuals and of most aquatic Dicotyledons. 



The cambium in roots is always definitely a secondary structure, that 

 is to say, it is not traceable directly to the apical meristem, but arises by the 

 alteration of mature parenchyma cells. It appears at first in a number of 

 disconnected arcs around the inner margins of the phloem groups (Figs. 

 800 and 801). 



We shall consider the details of cambial growth under the stem (see p. 

 891), so at present we may simply say that these cambial strips actively 

 produce new, secondary xylem cells on their inner sides and a relatively small 

 number of secondary phloem cells on their outer sides. But whereas these 

 phloem cells are contiguous to the primary phloems, the secondary xylem lies 

 between the primary xylems, and on the same radii as the phloems. 



The cambial arcs soon extend laterally until they touch the pericycle at 

 their extremities. At these points the pericycle cells become meristematic 

 and form links round the outside of the protoxylems, thus uniting the separate 

 arcs of the cambium into a continuous band, running inside the primary 

 phloems and outside the primary xylems. 



By continued and rapid formation of secondary xylem inwards, the con- 

 cave arcs of the cambium zone move outwards, carrying the phloems with 

 them until the band ceases to be sinuous and becomes a perfect circle. 



Opposite the primary xylems the cambium only forms parenchyma cells, 



