SECRETORY PASSAGES 



523 



endodermal layers that are principally mechanical in function. These 

 mechanical sheaths are very characteristically developed in the leaves 

 of Pinus, and in the roots of Philodendron. In Pinus excelsa the cells 

 of the sheath are flattened in the tangential plane, and their walls are 

 but slightly thickened. In P. Ccmbra there is no tangential flattening : 

 the walls are thickened all round, and the cells are appreciably 





Fig. 212. 



Resin- and Oil-passages. A. Resin-passage from the leaf of Pinus sylveslris, in T.S. 

 B. Resin-passage from the leaf of Pinus sylvestris, in R. L.S. ; s, sheathing layer; 

 z, secretory cells. C. Oil-passage from the pith of the stem of Heracleum sphondyliiim 

 (T.S.) ; z, secretory cells. D. Resin-passage from the primary leptome of the stem of 

 Elms Cotinus (T.S.); s, sheathing layer ; :, secretory cells. 



elongated (being six to eight times longer than their width), and here 

 and there show a tendency to become prosenchymatous. Finally, in P. 

 Laricio, P. sylvestris (Fig. 212 a) and P. Strobus the cells of the sheath 

 resemble typical stereides in all essentials, being very thick-walled 

 and strongly prosenchymatous. The sheaths may become two-layered 

 at certain places. A further point of agreement with endodermal layers 

 consists in the presence of isolated or longitudinally seriated " passage- 

 cells," which are conspicuous on account of the thinness of their walls. 

 The provision of passage-cells is quite comprehensible in view of the 



