CUCURBITA 



609 



This and other investigations indicate that the vascular bundle 

 of the Cucurbitaceae should be regarded as strictly bicollateral, 

 if the criteria for that type as outlined by Herail (17) are accepted. 

 These are the differentiation of the inner phloem from the pro- 



FiG. 316. Transections showing stages in differentiation of hypocotyledonary vascular 

 bundle: A, provascular strand in embryo just prior to germination; B, vascular bundle of 

 three-day seedling showing development of outer protophloem ; C, vascular bundle of seven- 

 day seedling in which primary differentiation is nearly complete, with crushed protophloem 

 elements, fibers, metaphloem, inner phloem, and connective phloem which is conspicuous 

 because of dark staining cells : cci, companion cells of inner phloem ; can, companion cells of 

 metaphloem; co, cortex; dsc, dark staining cells in connective phloem; en, endodermis; 

 /■ ph, inner phloem ; mx, metaxylem ; pel, pericycle ; pi, pith ; pph, protophloem ; pp//, paren- 

 chyma cells of protophloem ; px, protoxylem ; src, sieve tubes of connective phloem ; .m, 

 sieve tubes of inner phloem ; stm, sieve tubes of metaphloem. (After Whiting.) 



cambial strand and the simultaneous development of the inner 

 and outer phloem. 



Secondary Thickening of the Root. — Secondary thickening 

 of the root is initiated by the differentiation of a cambium in the 

 zone of fundamental parenchyma betw^een the primary xylem and 



