INTERCELLULAR SECRETORY RESERVOIRS. 449 



section, and is bounded externally by two cells of the pericambial ring, internally by 

 three cells of the phloem group. The whole number of the passages in one root-bundle 

 may accordingly be very large, e.g. 2 x n next the xylem, plus 3 in the phloem, in the 

 diarch main root of Pastinaca ; 4 x 5 next the xylem, plus 4 in the phloem, in tetrarch 

 adventitious roots of CEnanthe pimpinelloides, &c. Adventitious roots like those of the 

 last-named plant may retain this primary structure for a long time, or even throughout 

 life. Usually, however, and in all investigated main roots, it is altered at an early stage. 

 The cambium which appears in the bundle (Sect. 139) produces externally a massive, and 

 for the most part parenchymatous secondary cortex ; the primary cortex, together with 

 the endodermis, is simultaneously thrown off with an abundant formation of periderm 

 (sect. 176) starting from the pericambial cells lying outside the passages, and strong 

 increase and division of the pericambial cells lying within the passages. The passages 

 thus come to lie near to the inner side of the periderm, at first with their original 

 arrangement, later, as the result of the increasing growth in thickness of the cortex, 

 more and more dislocated. They then represent the canals described by TrScul, which 

 lie under the peridermal covering of the roots of Umbelliferae. 



The hypocotyledonary stem of the seedling retains approximately the structure of the 

 bundles and arrangement of the passages of the root till close below the cotyledons. One 

 passage branches out from the hypocotyledonary stem to each of the three vascular 

 bundles which enter there, and has in the cotyledon a position close to and opposite the 

 phloem of the bundle. 



Though there are no definite statements on this point, it cannot be doubted that a 

 connection exists in the cotyledonary node between the passages hitherto described and 

 those found higher up in the stem. 



The arrangement of these differs in most cases from those hitherto treated of. 



In the outer cortex of the internodes there is as a rule one passage opposite each vascular 

 bundle, or each of the stronger ones (Fig. 193). Since one angle of the stem or one hypo- 

 dermal coUenchymatous bundle corresponds usually to each of the latter, especially the 

 stronger ones, the passages are thus, as Trecul states, also opposite the strands of collen- 

 chyraa. Instead of one only, 2, 3, or even 4 passages may be opposite to one broad 

 strand of coUenchyma. According to the species these passages are either near to the 

 periphery, close to or even in the strand of collenchyma ; or they have a more internal 

 position in the parenchyma between the latter and the vascular bundle. In the creeping 

 stem of Hydrocotyle vulgaris there is a passage, at the inner limit of the non-coUenchy- 

 matous cortex, closely opposite each vascular bundle, with its epithelium directly adjoin- 

 ing the inner face of the endodermis (p. 1 2 1) ; the same arrangement appears in the branches 

 of Bupleurum fruticosum. 



In addition to the passages mentioned, which may be called fascicular, there are in 

 many species more or less numerous ones situated in other positions in the outer cortex ; 

 thus e. g. according to Trgcul, in all regions of it, from the epidermis to the limit of the 

 ring of bundles in Smyrnium Olusatrum, ^gopodium Podagraria, and Sison Amomum. 

 Trecul distinguishes, according to -the- conditions of arrangement indicated, ten types, 

 the number of which may be increased if necessary. 



Almost all Umbelliferse have oil passages in the pith of the internodes of the stem ; but 

 Trecul cites Bupleurum Gerardi and ranunculoides as exceptions to this. Also in Hy- 

 drocotyle vulgaris and Xanthosia rotundifolia I find no medullary passages. The flower- 

 ing branches of Bupleurum fruticosum show, according to the same author, in the upper 

 internodes numerous medullary passages alternating with the inner margins of the vas- 

 cular bundles ; in the lower internodes the number of them diminishes successively, but 

 they seem, according to the description, which I cannot fully understand, to be present 

 originally also at the base of the branch, and to be crushed by secondary extension of the 

 surrounding cells of the pith. In the seedling of Foeniculum officinale the medullary 

 passages are often entirely absent in the first internodes (comp. Fig. 193); in higher in- 

 ternodes they appear quite isolated at first, but in large numbers in the stronger plant. 



