66 PRACTICAL BOTANY 



b. Parenchyma, by division of the cells by 



transverse septa. 



c. Fibres, by elongation and interweaving of 



the cells, while the width of the 

 individual cells is relatively reduced. 



Observe intermediate stages between cambium-cells and 

 these several mature tissues : this may best be done in sections 

 cut from stems in early summer. 



X. To investigate the nature of the crystals, several times 

 observed in the parenchyma of the stem of the Elm, cut 

 tangential sections of the phloem or of the cortical tissue, 

 mount in water, and having found one or more crystals 



(i.) Run some iodine solution under the cover-slip : the 

 crystal is not stained. 



(ii.) Acetic acid : it is not attacked. 



(iii.) Dilute nitric acid : it is more or less completely 

 dissolved. 



These reactions, coupled with what is known from the 

 analysis of ash, point to the conclusion that these are crystals 

 of calcium oxalate. 



