l82 



LECTURE XT. 



contain latex-like emulsions of resinous and gummy substances (De Bary). Oc- 

 casionally these secretion-bearing intercellular passages only form short gaps in 

 the tissue, as in the small leaves of the Cupressineas and in the seed-coats of some 

 plants of the same group. 



Fig. 182.— Resin canals in the young stem of the Ivy (transverse 

 boundarj' of tlie cambium (<:) and soft bast (-wb) ; /; wood. D and i 

 boundary between the bast (W and the cortical parenchyma (rf). 



.1, B. C young canals U^) . 

 nd older canals (^) lying 1 



The intercellular secretion-passages arise early in the tissue-differentiation in 

 young organs ; and may also be repeatedly formed in the secondary tissue-layers 

 produced from the cambium, especially in woody plants. It is generally by the 

 separation of the angles of four neighbouring longitudinal series of cells that the 

 passage is produced: this becomes filled at once with the secretion. If the passage 

 remains narrow in diameter, the cells surrounding it either remain undivided, or, 

 growing slightly, they undergo a single division only, so that the secretion-passage 

 is only bounded by a few cells on the transverse section. If, on the other hand, the 

 organ referred to increases in circumference generally, the secretion-passages running 

 in it may also be considerably extended in width ; the cells surrounding them grow- 

 to a corresponding degree with it,- and become divided by radial as well as by 

 tangential walls (taken with regard to the passages), so that the passage is now 

 surrounded by two, or even by several layers of a special tissue, the epithelium of 

 the passage, the granular contents and delicate wall of which distinguish it from 

 the surrounding tissue. In the leaves of species of Pinus, the bordering of the 

 epithelium of the passages by a closed sheath of schlerenchyma cells is very 

 characteristic. As regards the source of the secretion, there can be no doubt 

 that it originates from the epithelium; although it is not quite certain that the 



