BICOLLATERAL BUNDLES 



81 



and outer (Fig. 37) . Two groups of phloem occur in each bundle, 

 one external (Ph.'), separated from the xylem by the cambium 

 (Ca.), the other internal (Ph."), separated from the protoxylem 

 (P.xy.) by ordinary parenchyma. Such bundles, termed bicol- 

 lateral, are not uncommon in certain families of Flowering Plants, 



FIG. 37. Diagram of a transverse section of the stem of the Vegetable 

 Marrow (Cucurbita). Ca., cambium ; Col., collenchyma ; Ep., epi- 

 dermis ; M.xy., metaxylem ; Ph.', outer phloem ; Ph.", inner phloem ; 

 P.xy., protoxylem ; Scl., sclerenchyma (black). 



such as the Cucumber-family or Cucurbitaceae (to which the 

 Marrow belongs), and the Potato-family or Solanaceae, and offer 

 a marked contrast to the normal collateral type seen in the Sun- 

 flower. The elements of both xylem and phloem are remarkably 

 large, and are therefore especially suited for a study of their 

 detailed structure, which can be rendered more distinct by 

 staining with eosin (see Appendix VI). 

 6 



