INTERCELLULAR SECRETORY RESERVOIRS. 447 



separated from the plerome-sheath by a special layer, often consisting of numerous small 

 epithelial cells. In addition to those which traverse the primary cortex, there are in 

 certain species, but not all, other passages situated at the periphery of the pith, but 

 these are only to be seen above the cotyledonary node. 



As regards the special distribution in the transverse section of the stem van Tieghem 

 quotes the following special cases. 



(i) Only cortical passages are present in contact with the plerome-sheath. 



(a) Only one passage in the middle of the outer margin of each main leaf -trace bundle; 

 Senecio vulgaris, Kleinia ficoides, Cineraria maritima, Flaveria contrajerva, Bellis perennis, 

 Petasitps niveus, Baccharis halimifolia, &c. 



{b) The same, but in addition as many passages opposite the outer margin of each 

 united leaf-trace (faisceau reparateur) as single bundles of the trace coalesce above to 

 form the united trace : Aster. 



{/) On each side close to the phloem of each main bundle of the trace is one passage : 

 Tagetes patula. Arnica Chamissonis, Tanacetum vulgare, Cotula matricarioides, Ana- 

 cyclus Pyrethrum, Pyrethrum Parthenium, Santolina Chamsecyparissus, Achillea Mille- 

 folium, Zinnia elegans, Inula montana, Cirsium arvense, &c. 



(d) An unequal number, e. g. 3-5 passages opposite the outer margin of each main 

 bundle : Centaurea atropurpurea. 



(«■) A group of passages opposite each lateral margin of the phloem of each main bundle: 

 Silybum marianum. 



(2) Cortical and medullary passages are present, the latter opposite the xylem of the 

 bundles. 



(«) Medullary passages only opposite single bundles, e. g. two : Ageratum conyzoides. 



(b) Opposite each bundle of the leaf-trace is externally a cortical, and internally a 

 medullary passage : Solidago limonifolia. 



{c) Opposite each bundle of the leaf-trace one medullary and several cortical passages 

 occur : Serratula centauroides. Dahlia variabilis. 



(</) Near each bundle is one group of medullary and one of cortical passages : Carduus 

 pycnocephalus, Spilanthes fusca. 



(f) Opposite each bundle is a curved series of medullary and another of cortical pas- 

 Sages : Helianthus tuberosus. 



The petioles and leaves have no oil passages when they are absent in the stems bearing 

 them, while they are rarely wanting when the latter do contain passages : Xeranthemum 

 cylindraceum, Cirsium arvense, radical leaves of Lappa grandiflora. In most cases oil 

 passages are present in the leaves, and especially as direct continuations or branches 

 of cauline passages ; often also others occur which may be called accessory. The former 

 may, however, be called fascicular, since they accompany the bundles, either directly, like 

 the cortical ones, or in close proximity to the parenchymatous or endodermal sheath, which 

 each individual bund^^ takes with it from the stem into the leaf. In single plants, as 

 Tussilago Farfara and Cineraria maritima, they are intercalated in the sheath itself. 

 Their number and arrangement about each bundle, as seen in transverse section, differ 

 similarly according to the species, but still more variously than in the stem, as may 

 be seen from the examples quoted by van Tieghem, I.e., pp. 118 and 133, &c. They 

 either accompany the branches of the vascular bundles through the lamina (on this point 

 closer investigation is required), or they are limited to the midrib or rachis, as in the leaf 

 of Tagetes patula, where they do not enter the lateral segments of the leaf. Their form, 

 average size, and limitation are, at least at first, the same as in the root. 



In addition to these fascicular passages there are in single cases, in those species which 

 form passages in the secondary bast of the stem, others situated in the last-formed parts 

 of the phloem of the petiolar bundle : e. g. Helianthus annuus. 



Van Tieghem found accessory passages in the leaf of Solidago limoniifolia : beneath the 

 epidermis of the lower surface, and separated from it by 1-2 layers of collenchymatous 

 cells, there is a series of 3-5 narrow canals on either side of the mid-rib. Again, in 



