INTERCELLULAR SECRETORY RESERVOIRS. 



203 



smaller than that of the neighbouring parenchyma, so that they appear very different 

 from it in transverse sections : in rare cases they are distinguished from it by their greater 

 width (roots of Compositae, branches of many species of Rhus according to Trdcul). 

 Their inner surface is often slightly convex towards the passage ; in the mucilage- 

 passages of the Marattiaceae it is even elongated and conical ; in those of the leaves of 

 Lycopodium the cells bulge in a club-like manner. Where an epithelium can be 

 distinguished in isodiametric cavities (Lysimachia punctata, and its ally, Myrsine) 

 the cells are flattened towards the surface of the cavities. 



The wall of the epithehal cells is delicate, in resin- and balsam-passages often 

 coloured brown or yellow. In the mucilage-passages of old leaves of Cycas revoluta 

 alone it is stated by Trecul ' that they are strongly thickened on the side next the 

 passage. 



A 



c 



ApoScfc 



1^ 



ih^' 



Fig. 85.— Hedera Helix ; transverse sections of tlie young stem (800) ; j,' resin-passages. These are in A, B, C 

 j'oung, and have appeared between four and five rows of cells, in the secondary cortex 7ti i, at tlie limit of the 

 zone of thickening r; A wood ; in D and I: older, larger passages ; A bast ^ rp parenchyma of the outer cortex. 

 From Sachs' Textbook. 



There is a great lack of investigations on the protoplasm and contents of the 

 cells of the epithelium, and of the surroundings of the secretory reservoirs generally. 

 It seems to be certain for all cases that they contain small masses of the secretion, 

 which then filter in some wav in mass through the membrane into the reservoir. The 

 cells surrounding the young oil passages in the roots of Compositae have clear 

 contents, in which, in Helianthus annuus ^, large quantities of tannin are shown to be 

 present by reagents ; this is also the case with the oil in the passages. In Tagetes 

 patula ^, at the transition from the root to the hypocotyledonary stem, a clear violet 



' I-'Institiit, 1862, p. 315. 

 ' Sachs, Botan. Zeitg. 1859, p. 183. 



^ Van Tieghem, /.c. p. 113. 



