36 THE STRUCTURE OF ECONOMIC PLANTS 



cies of the Moraceae; and, in Cannabis, they develop as un- 

 branched, unicellular vessels which are multinucleate. The vessels 

 originate in young leaves at the tips of the vegetative axis, and 

 develop in the periphery of the primary phloem, attaining an 

 extraordinary length in some instances. Latex vessels of the 

 articulated type in which a longitudinal series of elongated cells 

 coalesce to form a continuous tube occur in Ipomoea and Lactuca. 

 In the latter, the ducts at first consist of a longitudinal series of 

 cells with definite end walls; but, later in ontogeny, they become 

 continuous non-septate passages, and frequent cross-anastomoses 

 result in a system of interconnecting canals. (See Fig. 334.) In 

 both Ipomoea and Lactuca, they develop primarily in the phloem 

 and pericyclic tissues; but, in the latter, they also occur in the 

 cortical region of the axis and the mesophyll of the leaf. 



LITERATURE CITED 



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 1. , and Kerr, T., "Growth and structure of cotton fiber." Jour. Ind. 



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