208 



BOTANY AND PHARMACOGNOSY. 



the hypodermal strata of the cortex or in still other cases we find 

 its development mucli deeper, even within the pericycle. 



In regard to the increase in thickness, the stem behaves much 

 like the root, as in the throwing off the peripheral tissues extend- 

 ing from the epidermis to the endodermis, or only of the epidermis 

 and adjoining cortex, which then becomes replaced by strata of 



Fig. 117. Transverse section of midrib of leaf of stramonium: EU, upper epidermis; 

 CO, collenciiyma; PA, palisade cells; O, layer of cells containing rosette aggregates of 

 calcium oxalate; M, loose mesophyll; EL, lower epidermis; OP, prisms of calcium oxalate; 

 OS, cryptocrystalline crystals of calcium oxalate; ST, stoma; T, ducts; SU, sieve on upper 

 side of ducts; SL, sieve on lower side of ducts, this arrangement of sieve and ducts forming 

 bicollateral fibrovascular bundles. 



cork and secondary cortex. The mestome strands in the stem, 

 however, grow in a more regular manner than is the case with 

 those of the root, as is seen in the very distinct and freciuently 

 very regular layering of the tissues of woody stems, forming the 

 so-called " Annual Rings," where each ring represents the growth 

 that occurs during a single year. The development of these 

 annual rings depends especially upon the fact that the growth 

 of the perennial stem does not take place continuously, but is in- 



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