524 SECRETORY AND EXCRETORY SYSTEMS 



fact that the raw material employed in the manufacture of the resin has 

 to he conveyed to the secretory cells from the adjoining photosynthetic 

 parenchyma. As a matter of fact, the passage-cells may apparently 

 undergo a considerable amount of thickening in old leaves, when the 

 secretory cells have ceased to be active. Similar thick-walled often 

 many-layered mechanical sheaths are found surrounding the secretory 

 ducts in the roots of Philodendron. The oil-passages that traverse the 

 primary leptome of Rhus Cotinus are likewise encased in a double 

 sheath of cells, which are, however, thin-walled and flattened 

 (Fig. 212 D). 



Some account must next be given of the course and arrangement of 

 secretory passages. The simplest scheme is that in which an organ is 

 traversed by longitudinal secretory passages which are closed at both 

 ends ; this condition is exemplified by the acicular leaves of Pinus 

 sylvestris, P. montana and P. Cembra, and probably, also, by those of 

 many other Abietineae. A Pi?ius-needle contains a variable number 

 of sub-epidermal resin-ducts. There are always two principal lateral 

 passages, and from two to twenty accessory ducts on the abaxial and 

 adaxial sides of the needle. The two lateral passages extend further 

 towards the base of the leaf than the rest. At a distance of 2" 5 mm. 

 above the leaf-base, the glandular tissue of each lateral duct is reduced 

 to three or four cells (as seen in transverse section), which, however, 

 still form a continuous cylinder ; half a millimetre from the base the 

 secretory cells disappear altogether, and the resin-passage is replaced 

 by a sharply- defined fibrous strand corresponding to the contracted 

 thick-walled sheathing layer. The median ducts terminate in a similar 

 manner ; the intercellular cavity disappears first, and the secretory cells 

 follow suit later, so that finally nothing remains but a subepidermal 

 strand of fibres. These accessory ducts, however, come to an end 

 sooner than the lateral passages, their secretory cells disappearing at a 

 distance of 5 to 7 mm. from the leaf-base. The leaf of Junipcrus 

 communis contains a single median abaxial passage, which ends blindly 

 close to its base ; the wide passages which occupy the three angles 

 of each internode, likewise end blindly above and below. 2 " 9 



In the majority of cases, the secretory passages form a branched 

 anastomosing system of tubes pervading the whole plant-body. As a 

 rule, this tubular system is most abundantly developed in the paren- 

 chymatous tissues, but not infrequently the vascular bundles also 

 contain secretory ducts. The mucilage-ducts of Mauattiaceae and 

 Cycadaceae, the resin-passages of Conifers, and the oil-canals of the 

 tubifloral Compositae and the Umbelliferae, all occur principally in the 

 cortical and medullary parenchyma of the stem. In some species of 

 Araucaria, in certain Clusiaceae, and in the Anacardiaceae (Bhus 



