1960] 



MORPHOLOGY AND RELATIONSHIPS OF DIOMMA 



45 



the formation of a sclerenchymatous zone exterior to (part of J) the phloem 

 and becoming visible 6 or 8 mm distal to the petiolar attachment. This zone 

 first develops in the abaxial region as an arc of sclerenchyma, and as one pro- 

 ceeds distally, the arc extends until it surrounds the vascular cylinder. No 

 medullary vascular bundles were observed in the pith of the petiole of Diomma. 



SECRETORY STRUCTURES 



Secretory structures in the tissues of plants can be divided into two general 

 categories : ( 1 ) the secretory cell which is idioblastic in nature and is not con- 

 tiguous with other such cells, and (2) the secretory intercellular space which is 

 surrounded by a continuous layer of cells composing a secretory epithelium. 



Fig. 3. A, Sohnreyia excelsa, diagrammatic representation of secretory cavity from leaf 

 to show association with vascular tissue and illustrate the general nature of these structures 

 in the taxa under consideration. B, 1-4, Diomma ulei, serial sections of petiole from lower 

 pulvinus at 1 to upper pulvinus at 4, demonstrating the formation of a vascular cylinder from 

 originally distinct vascular strands at 1 (a — vascular tissue, b — sclerenchymatous zone). 



