LATICIFEROUS VESSELS 



203 



Interior tubular or canalicular glands are formed in essen- 

 tially the same manner as the globular. A circular group of 

 cells as seen in cross section, and a long vertical row as seen in 

 longitudinal section, forms an intercellular space schizogen- 

 ously or lysigneously at its center throughout its length. If 

 formed schizogenously the secreting cells form a sheath around 

 the intercellular canal as seen in Fig. 113, and into this canal 



f 



f 



Fig. 113. — Resin duct in leaf of Pinus silvestris, in cross section at A, and in longi- 

 tudinal section at B; h, cavity surrounded by the secreting cells;/, /, sclerenchyma fibers 

 surrounding and protecting the duct. (After Haberlandt.) 



are excreted the secretions of the sheath cells. Fine examples 

 of tubular canals are found in the needles and stems of pines. 



In the few instances where the tubular glands are differen- 

 tiated from procambium strands they have the same method 

 of formation as have those from the ground meristem. The 

 tubular glands often branch and anastomose and thus form 

 a complex glandular system. 



Laticiferous Vessels or Milk Tubes. — The milk tubes 

 occurring in many families of plants form a much-branched, 

 richly anastomosing system extending practically throughout 

 the whole plant (Fig. 114). They have two methods of origin. 

 In the Lobeliaceae, Cichoriaceae, Papaveracese, Campanulaceae, 

 Papayaceae, and a few Euphorbiaceae, and many Musacese and 

 Aroidese they are formed by cell fusions which take place early 

 in the primary meristematic condition by digestion of separating 

 walls. The anastomoses which unite the tubes into practically 



