44<5 PRTAfARF ARRANGEMENT OF TISSUES. 



contents) of oil-containing passages. There they together form the curved series, and 

 are separated from one another laterally by a single cell only. According to van Tieghem 

 the oil passages, especially in Tagetes patula, arise at the point indicated, between con- 

 centric layers of cells, which are derived by tangential division of the originally simple 

 endodernial layer, the latter remaining simple opposite the plates of xylem. In the 

 seedling of Helianthus annuus both cases may be found side by side in the same main 

 root. In addition to these normal passages there are not uncommonly other peripheral 

 ones, which arise in the same way between the layer surrounding the endodermis and 

 the next outer layer. All the passages are at first narrow, the two lateral and outermost 

 ones of each normal curved series are triangular, the rest quadrangular. When a peri- 

 pheral series is present, its members may coalesce with those of the inner series by 

 splitting of the cell-wall separating them. The number of the passages of any normal 

 curved series or group shows individual variations even in one and the same root, but 

 comparison of a number of cases shows that there are different average numbers which 

 are characteristic for groups, genera, and species. 



These numbers are highest in those Cynarese which have been investigated — the 

 following quotations refer in each case to a single group opposite one mass of phloem; — 

 there are lo and more in the diarch main roots of Carduus pycnocephalus, Silybum 

 ^ marianum, Xeranthemum cylindraceum, the diarch or triarch roots of Centaurea atro- 

 purpurea, Echinops exaltatus ; 15-20 in the diarch main root of Cirsiuin arvense ; 12-15 

 in the tetrarch subsidiary roots of Serratula centauroides. Calendula officinalis has 

 8-10 ; Venidium calendulaceum 3-5. 



In the Senecioneae the number decreases; e.g. Helianthus annuus, in the tetrarch 

 main root, 5-8 ' ; Gnaphalium in the diarch root, 5-8 ; Tagetes patula, in the diarch 

 main root, 5-7; Arnica Chamissonis, Tanacetum vulgare, tetrarch, 4-6; Cotula matri- 

 carioides triarch, 2 ; Achillea millefolium the same, 1-3 ; Senecio vulgaris, tetrarch, 2, 

 sometimes united into one ; Pyrethrum Parthenium, triarch, i, rarely 3, &c. 



Among the Astereae van Tieghem found 6-8 in a triarch root of Inula montana, but 

 only one in Bellis perennis, Erigeron glabellus. Aster, Conyza, and species of Solidago. 

 In the latter cases, especially in Solidago limoniifolia, the canal may be greatly widened, 

 by the separation of the cells which originally limited it externally, so that it extends 

 as far as the next outer layer, or even continues, by further separation of cells, into 

 several layers which lie further out. 



Among the Eupatorieae a triarch root of Tussilago Farfara showed 5-7 passages ; a 

 similar one of Ageratum conyzoides had 2-3 ; Petasites niveus and Eupatorium aroma- 

 ticum have i each, which is extended as in Solidago. 



As the primary cortex is stretched by the secondary formation of wood and bast, the 

 primary passages of the root, at least in the investigated Senecioneae, remain in their 

 place, while they increase to a variable extent in width, and the cells surrounding them 

 in number by division. Compare the details on Tagetes patula in van Tieghem /. c. and 

 the drawing of Radix Arternisiae in Berg. Atlas, Taf. XV. 



In the stem of the Compositae in question the oil passages are only absent in relatively 

 few exceptional cases: Echinops exaltatus, Gnaphalium citrinum, according to van 

 Tieghem. In by far the most numerous cases they are continuous from the root through 

 the stem with its branches and leaves, but subject to branchings, or increase in numbers, 

 which will be described below. They are seated in the primary tissue of the stem 

 always in close contact with the outer side of the plerome sheath, which in the Compo- 

 sitae (p. 415) may be followed from the hypocotyledonary portion through the whole stem, 

 covering the outside of the ring of vascular bundles. In the hypocotyledonary stem 

 the passages have the same structure and arrangement as in the root ; higher up, and 

 especially from the cotyledonarj^ node onwards, they change their arrangement, accord- 

 ing to that of the vascular bundles, in a niannef to be mentioned immediately; they are 



Compare Sachs, Botan. Zeitg. 1859, Taf. VIII. Fig. 7. 



