Читать книгу Lichens - Annie Lorrain Smith - Страница 64

Such structures are almost wholly confined to the larger foliose and fruticose lichens and are all of the same simple type; they are fungal in origin and very rarely are gonidia associated with them.

Fig. 52. Usnea florida Web. Ciliate apothecia (S. H., Photo.).

a. Cilia. In a few widely separated lichens stoutish cilia are borne, mostly on the margins of the thallus lobes, or on the margins of the apothecia (Fig. 52). They arise from the cortical cells or hyphae, several of which grow out in a compact strand which tapers gradually to a point. Cilia vary in length up to about 1 cm. or even longer. In some lichens they retain the colour of the cortex and are greyish or whitish-grey, as in Physcia ciliaris or in Physcia hispida (Fig. 110). They provide a yellow fringe to the apothecia of Physcia chrysophthalma and a green fringe to those of Usnea florida. They are dark-brown or almost black in Parmelia perlata var. ciliata and in P. cetrata, etc. as also in Gyrophora cylindrica. The fronds of Cetraria islandica and other species of the genus are bordered with short spinulose brown hairs whose main function seems to be the bearing of “pycnidia” though in many cases they are barren (Fig. 128).

Superficial cilia are more rarely formed than marginal ones, but they are characteristic of one not uncommon British species, Parmelia proboscidea (P. pilosella Hue). Scattered over the surface of that lichen are numerous crowded groups of isidia which, frequently, are prolonged upwards as dark-brown or blackish cilia. Nearly every isidium bears a small brown spot on the apex at an early stage of growth. Similar cilia are sparsely scattered over the thallus, but their base is always a rather stouter grey structure, which suggests an isidial origin. Cilia also occur on the margin of the lobes.

As lichens are a favourite food of snails, insects, etc., it is considered that these structures are protective in function, and that they impede, if they do not entirely prevent, the larger marauders in their work of destruction.

Fig. 53. Rhizoid of Parmelia exasperata Carroll (P. aspidota Rosend.). A, hyphae growing out from lower cortex × 450. B, tip of rhizoid with gelatinous sheath × 335 (after Rosendahl).

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.

Fig. 54. Peltigera canina DC. (S. H., Photo.).

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.”

Lichen rhizinae, distinguished by Reinke[367] as “aerial rhizinae,” are more or less characteristic of all the species of Parmelia with the exception of those belonging to the subgenus Hypogymnia in which they are of very rare occurrence, arising, according to Bitter[368], only in response to some external friction. They are invariably dark-coloured, rather short, about one to a few millimetres in length, and are simple or branched. The branches may go off at any angle and are sometimes curved back at the ends in anchor-like fashion. The Parmeliae grow on firm substances, trees, rocks, etc., and the irregularities of their attaching structures are conditioned by the obstacles encountered on the substratum. Not unfrequently the lobes are attached by the rhizinae to underlying portions of the thallus.

In the genus Gyrophora, the rhizinae are simple strands of hyphae (G. polyrhiza) or they are corticate structures (G. murina, G. spodochroa and G. vellea). They are also present in species of Solorina, Ricasolia, Sticta and Physcia and very sparingly in Cetraria (Platysma).

c. Haptera. Sernander[369] has grouped all the more distinctively aerial organs of attachment, apart from rhizinae, under the term “hapteron” and he has described a number of instances in which cilia and even the growing points of the thallus may become transformed to haptera or sucker-like sheaths.

The long cilia of Physcia ciliaris occasionally form haptera at their tips where the hyphae are loose and in active growing condition. Contact with some substance induces branching by which a spreading sheath arises; a plug-like process may also be developed which pierces the substance encountered—not unfrequently another lobe of its own thallus. The long flaccid fronds of Evernia furfuracea are frequently connected together by bridge-like haptera which rise at any angle of the thallus or from any part of the surface.

The spinous hairs that border the thalline margins in Cetraria may also, in contact with some body—often another frond of the lichen—form a hapteron, either while the spermogonium, which occupies the tip of the spine, is still in a rudimentary stage, or after it has discharged its spermatia. The small sucker sheath may in that case arise either from the apex of the cilium, from the wall of the spermogonium or from its base. By means of these haptera, not only different individuals become united together, but instances are given by Sernander in which Cetraria islandica, normally a ground lichen, had become epiphytic by attaching itself in this way to the trunk of a tree (Pinus sylvestris).

In Alectoria, haptera are formed at the tip of the thallus filament as an apical cone-like growth from which hyphae may branch out and penetrate any convenient object. A species of this genus was thus found clinging to stems of Betula nana. Apical haptera are very frequent in Cladonia rangiferina and Cl. sylvatica, induced here also by contact. These two plants, as well as several species of Cetraria, tend, indeed, to become entirely epiphytic on the heaths of the Calluna formations. Haptera similar to those of Alectoria occur in Usnea, Evernia, Ramalina and Cornicularia (Cetraria). In Evernia prunastri var. stictoceros, a heath form, the fronds become attached to the stems and branches of Erica tetralix by hapteroid strands of slender glutinous hyphae which persist on the frond of the lichen after it is detached as small very dark tubercles surmounted, as Parfitt[370] pointed out, by a dark-brown grumous mass of cells. Plug-like haptera may be formed at the base of Cladoniae which attach them to each other and to the substratum. The brightly coloured fronds of Letharia vulpina are attached to each other in somewhat tangled fashion by lateral bridges or by fascicles of hyphae dark-brown at the base but colourless at the apices, exactly like aerial adventitious rhizinae. They grow out from the fronds generally at or near the tips and lay hold of a neighbouring frond by means of mucilage. These haptera are evidently formed in response to friction. Haptera along with other lichen attachments have received considerable attention from Galløe[371]. He finds them arising on various positions of the lichen fronds and has classified them accordingly.

After the haptera have become attached, they increase in size and strength and supply a strong anchorage for the plant; the point of contact frequently forms a basis for renewed growth while the part beneath the hapteron may gradually die off. Haptera are more especially characteristic of fruticose lichens, but Sernander considers that the rhizinae of foliose species may function as haptera. They are important organs of tundra and heath formations as they enable the lichens to get a foothold in well-lighted positions, and by their aid the fronds are more able to resist the extreme tearing strains to which they are subjected in high and unsheltered moorlands.