FORMS AND SFSTEMS OF TISSUES. 



93 



development the whole substance of the gland now becomes, as it were, forced outwards 

 above the surface of the organ (C) ; and when, finally, the secreting tissue is absorbed, 

 a cavity is formed filled with mucilage and drops of volatile oil, and surrounded 

 only by the continuation of the epidermis. Similar to glands in their origin are the 

 gum-passages and gummy swellings of diseased stone-fruit. Gregorieff found the seat 

 of the formation of the gum in them to ba principally the soft bast of the fibro-vascular 

 bundles which traverse the fruit-pulp ; the cell-walls become absorbed after they have 

 swelled up, and cavities with undefined boundaries filled with gum are thus formed, 

 which sometimes exude their contents externally through the flesh of thi fruit when 

 the production of gum is excessive. 



While the origin of the structures now described is the coalescence of cells previously 

 separated by partition-walls, the canal-like Receptacles for Secretions are formed, in many 

 plants, by cells, previously in contact by their partition-walls, separating from one. 

 another and leaving an intercellular space (in the manner represented in Fig. 66, p. 78), 

 into which the secretion flows from the surrounding cells. According to the nature of 

 the secretion, we thus get Resin-passages, as in most Coniferae and Terebinthaccce, 

 Gum-passages in Gycadeae, passages with a mixture of gum and resin in Umbelliferae 

 and Araliaceae ; Latex -pas sages in Rbus and in Alisma Plantago beneath the epidermis 

 of the petiole in front of the fibro-vascular bundles ; and passages with diff'erent vola- 

 tile oils, often coloured, as in the Compositae. Similar Secretion-canals ^ occur also in 

 Clusiaceae (the contents of the canals of Garcinia Morella yielding gamboge) and in 

 Pittosporeae ; among Monocotyledons, in addition to AUsma Plantago^ we find them 

 in Aroideae. Among Ferns, they are stated to have been detected in Marattia and 

 Angiopteris, and by Hegelmaier in some species of Lycopodium, as L. inundatum, alo- 

 pecuroides, and annotinum. Plants which possess secretion-canals have, as a rule, no 

 laticiferous vessels ; but they both occur in some Gichoriaceae as Scolymus, Gynaraceae 

 as Cirsium and Lappa, and in some Aroideae as Philodendron. They are in that case 

 distributed through different tissue-systems. Thus in Philodendron the laticiferous 

 vessels are found in the phloem of the fibro-vascular bundles, the secretion-canals in 

 the fundamental tissue ; while in Scolymus and Cirsium the laticiferous vessels run 

 through the phloem, the secretion-canals through the fundamental tissue of the 

 cortex^. Where there are only secretion-canals, they may belong exclusively to the 

 fundamental tissue of the primary cortex {Tagetes patula, according to Van Tieghem), 

 or exclusively to the phloem (the stem of Pittcsporum Tobira, according to Van 

 Tieghem), or to both systems (Umbelliferae) ; in Goniferae they occur in the pith 

 and primary cortex, and also in the phlo?m and xylem. Where they are found in 

 the secondary phloem formed out of the cambium, the secretion-canals may be 

 produced repeatedly with other elements of the soft bast in concentric layers, as in 

 Cussonia, Umbelliferae, &c. 



The simplest forms of secretion-canals are produced by three, or more commonly 

 fouf, longitudinal rows of secreting cells separating from one another far enough to 

 form a narrow intercellular passage which they fill with their secretion, as, for example, 

 in the roots of Gompositae, where definite groups of canals of this nature are formed 

 in the double vascular bundle-sheath. If the tissue which surrounds the secretion- 

 canal attains a vigorous development in breadth and thickness, the intercellular pas- 

 sages, which were at first narrow, become considerably broader (Fig. 66, p. 78), while 

 the secreting cells which surround them also enlarge, divide in radial and tangential 

 directions (in reference to the centre of the canal), and thus form round the canal 



^ See especially Van Tieghem, Les canaux secreteurs des plantes, Ann. des Sci. Nat., 5th ser. 

 vol. XVI, 1872.— Mliller in Jahrb. flir wiss. Bot. vol. V. p. 387.— Thomas, ibid. vol. IV. pp. 

 48-60. 



^ Van Tieghem, in the French translation of the 3id ed. of this work, p. 158. 



