200 



MECHANICAL SYSTEM 



regions of this composite strand, this process ceases entirely at an 

 early stage of development in the constricted middle zone : here the 

 cells expand, undergo repeated transverse divisions, and so become 

 gradually converted into parenchymatous tissue. In consequence of 

 this peculiar behaviour, the primarily continuous procambial strand 

 becomes separated into two parallel bundles which complete their 

 development quite independently of one another, the outer one be- 

 coming a subepidermal fibrous strand, while the inner one gives rise 



m, 



Fig. 72. 



Development of stereome from procambium. A. Part of the adaxial half of a T.S. 

 through a young leaf of Pandanui utilis ; c, procambial strands which will give rise 

 to small fibrous strands. B. Part of a radial longitudinal section through the same 

 leaf; c, as in A. x400. C. Part of a T.S. through a young petiole of Asplenium 

 foecundum, xl2o; c, procambial cylinder which will give rise to the subepidermal 

 fibrous cylinder. (In all figs. p = protoderm, and m = fundamental meristem.) 



to the mestome-bundle together with its two semi-lunar fibrous sheaths. 

 In some Caryophyllaceae (Lychnis, Melandryum, Dianfhus, Saponarid) 

 the fibrous cylinder and the mestome-bundles likewise arise from 

 a common procambial cylinder, which becomes secondarily separated 

 into an external cylinder of bast -procambium, an internal circle of 

 ribbons and strands representing the mestome-procambium and an 

 intermediate zone of parenchyma. 



According to Ambronn, collenchyma and mestome may similarly 

 originate from a common primary meristem and only become second- 



