144 THE STRUCTURE OF ECONOMIC PLANTS 



is some development of secondary vascular tissues. Latex ducts 

 occur in the primary phloem and extend into the stem without 

 branching. As in the stem, numerous crystals of calcium oxalate 

 are found in the parenchymatous tissue of the mesophyll and in the 

 phloem parenchyma of the veins. 



The petiole is subtriangular in transection with a groove extend- 

 ing along its adaxial surface which is shallow near the base of the 

 petiole and becomes gradually deeper toward the point of diver- 

 gence of the leaflets. It is strengthened by a zone of collenchyma 

 extending completely around the periphery adjacent to the epider- 

 mis, which is somewhat thicker along the abaxial angle and lateral 

 margins. The remaining tissue of the petiole consists of chloro- 

 phyll parenchyma, except for the veins. There are three at the 

 base of the petiole, but these usually anastomose in the upper por- 

 tion of the petiole, forming a continuous crescentic zone from which 

 branches arise that extend into the leaflets. 



A situation has been reported regarding the xylem of the petiolar 

 bundles which may be noted here. Alexandrov and Alexandrova 

 (i), in investigating the question as to whether or not lignification 

 of cell walls is an irreversible process, have pointed out that in 

 hemp the oldest xylem vessels in the lower petioles are regularly 

 delignified. According to their account, the walls of the vessels 

 are dissolved completely, even the cellulose, and the vessels are 

 replaced by proliferating parenchymatous cells. In their opinion, 

 this delignification may occur normally to a great extent and is not 

 necessarily connected with pathological or traumatic conditions. 



LITERATURE CITED 



I. Alexandrov, W. G., and Alexandrova, O. G., "1st die Verholzung ein 

 reversibler oder irreversibler Vorgang?" Planta y: 340-346, 192.9. 



1. DE Bary, a., Comparative Anatomy of the Vegetative Organs of the Phanerogams 

 and Ferns, Clarendon Press, Oxford, 1884. 



3. Berkman, a. H., "Seedling anatomy of Cannabis sativa L." Unpubl. Ph.D. 



Thesis, Univ. of Chicago, 1936. 



4. Bredemann, G., "Beitrage zur Hanfziichtung. III. Weitere Versuche iiber 



Ziichtung und Fasergehalt." Zeitschr. Pflanzensiicht . 12: 159-2.68, 1916. 



5. Breslawetz, L., "Polyploide Mitosen bei Cannabis sativa L." Ber. Deut. 



Bot. Gesell. 44: 498-502., 1916. 



6. Briosi, G., and Tognini, F., "Intorno alia anatomia della canapa (Cannabis 



sativa L.). Parte prima: Organi sessuali." Atti. 1st. Bot. Pavia, Ser. II, 

 }.- 91-109, 1894. 



