ELM. STEM. 81 



consisting of roundish cells, resembling cells of the 

 medullary rays in form. . 4 



2. Cambium from which all the other tissues are 

 derived, the cells of which have a prismatic form. 



Taking these cells as a starting point, the several 

 tissues above described are derived from them in the 

 following way : 



(i). Phloem. (a). Sieve-tubes, by lateral disten- 

 sion and conversion of the 

 oblique walls into sieve-plates. 

 (&). Parenchyma, by division of 



the cells by transverse septa, 

 (c). Fibres (sclerenchyma), by elon- 

 gation and interweaving of 

 cells, the width of the cells at 

 the same time being relatively 

 reduced. 



(ii). Xylem. (a). Vessels, by lateral distension, 

 and absorption of cell-contents, 

 and of the terminal walls. 

 (6). Parenchyma, by division of the 



cells 'by transverse septa, 

 (c). Fibres, by elongation and inter- 

 weaving 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 



G 



