FORMS AND SYSTEMS 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 bs 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 thj 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 Goniferse and Terebinthace-de, 
Gum-passages in Gycadeae, passages with a mixture of gum and resin in Umbelliferae 
and Araliacese ; Latex-passages in Rhus and in AUsma Plantago beneath the epidermis 
of the petiole in front of the fibro-vascular bundles ; and passages with different vola- 
tile oils, often coloured, as in the Gompositae. Similar Secretion-canals ^ occur also in 
Glusiacese (the contents of the canals of Garcinia Morella yielding gamboge) and in 
Pittosporege ; 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 Gichoriacese 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 2. 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 Pittosporum Tobira, according to Van 
Tieghem), or to both systems (Umbelliferae) ; in Goniferae they occur in the pith 
and primary cortex, and also in the phloem 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 
four, 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 Sei. Nat., 5th ser. 
vol. XVI, 1872. — Müller in Jahrb. für wiss. Bot. vol. V. p. 387.— Thomas, ibid. vol. IV. pp. 
48-60. 
^ Van Tieghem, in the French translation of the 3rd ed. of this work, p. 158. 
