303 INTERCELLULAR SPACES. 



most prominently in Myrsine africana, and many species qf Lysimachia, where 

 the characteristic red resinous secretion occurs in sacs in the root, and in roundish 

 intercellular cavities in the other parts of the plant. 



The reservoirs in question may be arranged, according to their form and the 

 nature of the secretion, in the following systematic groups. 



1. Mticilage- and gum-passages in the Marattiacea, many Lycopodia, the Cycads, 

 species of Canna, and Opuntia, and some AraliacecB. Mucilage-containing cavities 

 of lysigenetic origin occur in single cases, mentioned above among sacs (p. 144). 



2. Resin, ethereal oil, emulsions oi gum-resin of different quality, according to the 

 special case, and often little known as regards chemical relations, occur — 



(a) in passages in the Conifer a, Alismacece, and Aroidea, the tuhifloral Composiice, 

 Umhellifera, Araliacece, PitiosporecB, many species of Mamillaria, Clusiacea, Anacar- 

 diacece, the genera Ailantus, and Brucea of the family Simarubese, 



(p) In short cavities in the group Ruiacea, in the sense of Bentham and Hooker 

 (exclusive of Simarubeae and Zygophylleae), in species of Hypericum, many species of 

 Oxalis, MyrtacecB, MyoporecB, species of Lysimachia, Ardisia, and Myrsine: perhaps 

 also in Gossypium. 



The wall of all secretory spaces, vs'hich is made up of that of the neighbouring cells, 

 is, with the exception of the above-mentioned special case of Lysimachia Ephemerum, 

 always completely closed, the lateral walls of the cells, i. e. those perpendicular to the 

 surface of the space, being joined together without interspaces. When, as is the case 

 in the petiole of Marattiacese and Cycadese, the original cells, which limit a passage, 

 are separated laterally from one another by continued growth of the surrounding 

 tissue in a peripheral direction, the enclosure is completed by the next outer layer of 

 parenchyma. The number of cells limiting the transverse section of a cavity varies 

 according to the particular case. The small resin-passages, which run transversely 

 between the larger longitudinal ones in the secondary cortex of Cussonia, are, 

 according to N. Miilier, at first at least, slit-like spaces between the partly separated 

 walls of two rows of cells, and are thus limited in transverse sections by two cells. 

 Most of these spaces appear in transverse section to be limited by 3, 4, or many 

 more cells, the number of which may increase as the space widens by divisions 

 placed radially with regard to the space. Fig. 85. 



The cells surrounding the secretory reservoir have generally the properties of 

 parenchymatous cells. In form they either do not differ, or only slightly, from the 

 cells of the neighbouring parenchyma, e. g. in the young roots of the Compositae, in 

 the leaves of Ardisia crenulata : or, as in most cases, they are sharply distinguished, 

 so that one might speak of a peculiar lining of the wall, or epithelium of the inter- 

 cellular space. According as the peculiar character extends' to one or several layers 

 of cells surrounding the space, there may be distinguished one-, two-, or several-layered 

 epithelium. The enclosure of the several-layered epithelium of the resin-passages in 

 Ihe leaves of Pinus Strobus, sylvestris, Laricio, &c., and of the roots of Philodendron, 

 in a completely, closed sheath of sclerenchyniatous fibres, which are wanting in the 

 homologous passages of other closely allied plants, is worthy of notice. The cells of 

 the epithelium are generally prismatic in elongated passages ; their greatest diameter 

 .usually lies in the direction of the length of the passage, only in the leaves of Cycads 

 (Kraus, /. c) do they lie transversely. Their transverse diameter is. .usually much 



