_b._ RHIZINAE. Lichen rootlets are mainly for the purpose of attachment and have little significance as organs of absorption. They have been noted in only one crustaceous lichen, _Varicellaria microsticta_[363], an alpine species that spreads over bark or soil, and which is further distinguished by being provided with a lower cortex of plectenchyma. In foliose lichens they are frequently abundant, though by no means universal, and attach the spreading fronds to the support. They originate, as Schwendener[364] pointed out, from the outer cortical cells, exactly as do the cilia, and are scattered over the under surface or are confined to special areas. Rosendahl[365] has described their development in the brown species of _Parmeliae_: the under cortex in these lichens is formed of a cellular plectenchyma with thickish walls; the rootlets arise by the outgrowth of several neighbouring cells from some slight elevation near the edge of the thallus. Branching and interlacing of these growing rhizinal hyphae follow, the outermost frequently spreading outwards at right angles to the axis, and forming a cellular cortex. The apex of the rhizoid is generally an enlarged tuft of loose hyphae involved in mucilage (Fig. 53), a provision for securing firmer cohesion to the support; or the tips spread out as a kind of sucker. Not unfrequently neighbouring “rootlets” are connected by mucilage at the tips, or by outgrowths of their hyphae, and a rather large hold-fast sheath is formed. [Illustration: Fig. 54. _Peltigera canina_ DC. (S. H., _Photo._).] [Illustration: Fig. 55. _Peltigera canina_ DC. Under surface with veins and rhizoids (after Reinke).] In species of _Peltigera_ (Fig. 54) the rhizinae are confined to the veins or ridges (Fig. 55); they are thickish at the base, and are generally rather long and straggling. Meyer[366] states that the central hyphae are stoutish and much entangled owing to the branching and frequent anastomosis of one hypha with another; the peripheral terminal branches are thinner-walled and free. These rhizinae vary in colour from white in _Peltigera canina_ to brown or black in other species. Most species of _Peltigera_ spread over grass or mosses, to which they cling by these long loose “rootlets.”