Laticifers and secretory ducts: two other tube systems in plants


  • 1

    A laticifer is thus named because it contains a latex. ‘Latex’ derives from poetical Latin and denotes a (frequently special) type of liquid; the Latin plural of ‘latex’ is ‘latices’. In its general botanical sense, latex is of highly variable chemical composition, is not necessarily of a milky appearance, and may contain precipitates of various sorts, or suspended colloids, or a variety of dissolved solutes (cf. Esau, 1965; Mahlberg, 1993). Moreover, because latex frequently is milky in appearance, a tissue that contains it may occasionally be termed ‘lactiferous’ (e.g. Haug et al., 2005); and here the ‘lact’ comes from a Latin root denoting breast milk. This author inclines toward the view that, if a tissue is known to contain laticifers in the sense of Esau (1965), it is certainly laticiferous, whereas, if a tissue is known only to exude upon puncture a whitish liquid, then it is still laticiferous because its exudate (however stored in the tissue) is certainly not breast milk.

  • 2

    Symplastic transport itself is more a distributed mechanism than one concentrated in a supracellular anatomical feature (i.e. tube), and it was therefore placed beyond the scope of this survey. However, recent relevant reviews have treated the significance of plasmodesmograms (Van Bel & Oparka, 1995); the role of plasmodesmata in symplastic transport (Roberts & Oparka, 2003); long-distance transport in nonvascular plants (Raven, 2003); the role of cytoplasmic streaming in symplastic transport (Pickard, 2003); and macromolecular trafficking (Oparka, 2004).

  • 3

    With the flowering of molecular taxonomy, the structure of taxonomic trees underwent and is still undergoing a period of great change. In this paper the tendency is to adhere to traditional nomenclature but within a framework of the tree structure continuously updated by the Angiosperm Phylogeny Website ( hosted by the Missouri Botanical Garden. The URLs and are also of value.

  • 4

    Exactly how to differentiate these plant products from one another is something of a judgement call (cf. Nair, 1995). However, polysaccharides are generally the prominent component in a gum. A gum will dissolve in (or form a suspension with) water to yield a viscous (and often adhesive) jelly, paste, or syrup. Gums are insoluble in organic solvents of low dielectric constant. Resins, in contrast, are classified as aromatic because of the prominence of ring compounds (often heavy and nonvolatile) in them. Resins are not water soluble but dissolve readily in organic solvents of low dielectric constant. Essential oils are lower molecular weight aromatic compounds of a distinctive odor (essence) which characterizes their source; they are a common constituent of resins.

  • 5

    In modern usage, the ‘apoplast constitutes all compartments beyond the plasmalemma’ (Sattelmacher, 2001, p. 167) while, by exclusion, the symplast seems to include everything within the plasmalemma. This view, while formally correct, does not encompass the normally unvoiced expectation that both these spaces should be filled with electrolyte of unremarkable properties; and a laticifer vacuole filled with rubber particles or a duct loaded with terpenoids may require rethinking some generalizations.

Author for correspondence: William F. Pickard Tel: +1 314 935 6104Fax: +1 314 935 7500Email:


The plant kingdom has elaborated several conducting systems. Three are primarily for mass transport: the aerenchyma (for gas exchange in submerged parts), the phloem (for exchange of nutrients within the plant), and the xylem (largely for transport of water from soil to transpiring leaves). Two others are believed to be primarily defensive and to store under pressure aversive contents which they exude when punctured: the laticifer and the secretory duct. This review provides for the latter two systems the highlights of what is known about their general physiology and ecophysiology but not their metabolism and their molecular biology. It is argued that, given the importance of laticifers and secretory ducts to plant defense against insect herbivory, these structures are under-investigated and deserve more intensive study.