PROCAMBIAL STEREOME 201 



arily separated ; this arrangement is of very widespread occurrence 

 in the Aroideae, Umbelliferae and Piperaceae. 



Where, as so frequently occurs, the strands of mechanical tissue and 

 the mestome-bundles are associated to form " fibro-vascular " bundles, 

 they almost always originate from a common meristem, consisting 

 either of a homogeneous procambial cylinder or of a series of homo- 

 geneous procambial strands, within which mechanical and conducting 

 elements become differentiated in accordance with the varying require- 

 ments of the plant. Very often longitudinal division continues for 



N a 



Fig. 73. 



Mode of origin of the fibrous cylinder and the internally apposed vascular bundles 

 in the scape of Primula sinensis (T.S.) ; 6, bast-procambium ; g, mestome-procambium 

 (primordia of the vascular bundles). 



a longer period in those portions of the common procambium which 

 are destined to give rise to vascular tissue ; this condition prevails, for 

 instance, in the scape of Prim ula sinensis, where the " primary " pro- 

 cambial cylinder gives rise to a fibrous cylinder, while the mestome- 

 bundles, which abut against the inner side of the latter, appear to 

 arise from "secondary" procambial strands (Fig. 73). 



Attention may finally be drawn to an important ontogenetic 

 distinction between stereome and mestome. The conversion of a 

 strand or strip of bast-procambium into permanent tissue takes place 

 simultaneously, or nearly so, over the whole of the cross-section. In 

 the case of mestome-procambium, on the contrary, the transformation 

 into permanent tissue normally begins first of all at two or more 



